Add support for JSON->proto conversion.

This is the basis for converting from GAE JSON structures to proto "v2"
(that is, the proto structures defined as part of oppia-proto-api). "v1"
conversion (that is, the proto structures used in Oppia Android's data
layer) will happen at a higher level in a downstream PR.

scripts/src/java/org/oppia/android/scripts/gae/proto/BUILD.bazel

@@ -1,8 +1,7 @@
+load("@io_bazel_rules_kotlin//kotlin:jvm.bzl", "kt_jvm_library")
 load("@rules_java//java:defs.bzl", "java_proto_library")
 load("@rules_proto//proto:defs.bzl", "proto_library")
 
-# TODO: The dependencies in this module are a bit broken with compat/. Needs refactor.
-
 proto_library(
     name = "extra_exploration_definitions_proto",
     srcs = ["extra_exploration_definitions.proto"],
@@ -14,3 +13,62 @@ java_proto_library(
     visibility = ["//scripts/src/java/org/oppia/android/scripts/gae:__subpackages__"],
     deps = [":extra_exploration_definitions_proto"],
 )
+
+kt_jvm_library(
+    name = "json_to_proto_converter",
+    testonly = True,
+    srcs = [
+        "JsonToProtoConverter.kt",
+    ],
+    visibility = ["//scripts:oppia_script_library_visibility"],
+    deps = [
+        ":extra_exploration_definitions_java_proto",
+        ":localization_tracker",
+        "//scripts/src/java/org/oppia/android/scripts/gae/json:model",
+        "//third_party:oppia_proto_api_java_protos",
+        "//third_party:org_jetbrains_kotlinx_kotlinx-coroutines-core",
+    ],
+)
+
+kt_jvm_library(
+    name = "localization_tracker",
+    testonly = True,
+    srcs = [
+        "LocalizationTracker.kt",
+        "OppiaWebTranslationExtractor.kt",
+    ],
+    visibility = ["//scripts:oppia_script_library_visibility"],
+    deps = [
+        ":image_downloader",
+        ":proto_version_provider",
+        "//scripts/src/java/org/oppia/android/scripts/gae/json:model",
+        "//third_party:moshi",
+        "//third_party:oppia_proto_api_java_protos",
+        "//third_party:org_jetbrains_kotlinx_kotlinx-coroutines-core",
+    ],
+)
+
+kt_jvm_library(
+    name = "image_downloader",
+    testonly = True,
+    srcs = [
+        "ImageDownloader.kt",
+    ],
+    visibility = ["//scripts:oppia_script_library_visibility"],
+    deps = [
+        "//scripts/src/java/org/oppia/android/scripts/gae/gcs:api",
+        "//third_party:org_jetbrains_kotlinx_kotlinx-coroutines-core",
+    ],
+)
+
+kt_jvm_library(
+    name = "proto_version_provider",
+    testonly = True,
+    srcs = [
+        "ProtoVersionProvider.kt",
+    ],
+    visibility = ["//scripts:oppia_script_library_visibility"],
+    deps = [
+        "//third_party:oppia_proto_api_java_protos",
+    ],
+)

scripts/src/java/org/oppia/android/scripts/gae/proto/ImageDownloader.kt

@@ -0,0 +1,55 @@
+package org.oppia.android.scripts.gae.proto
+
+import kotlinx.coroutines.CoroutineDispatcher
+import kotlinx.coroutines.CoroutineScope
+import kotlinx.coroutines.Deferred
+import kotlinx.coroutines.async
+import org.oppia.android.scripts.gae.gcs.GcsService
+import java.util.concurrent.ConcurrentHashMap
+
+class ImageDownloader(
+  private val gcsService: GcsService,
+  private val coroutineDispatcher: CoroutineDispatcher
+) {
+  private val imageLengths = ConcurrentHashMap<ImageId, Long>()
+
+  fun <T> retrieveImageLengthAsync(
+    imageContainerType: GcsService.ImageContainerType,
+    imageType: GcsService.ImageType,
+    entityId: String,
+    filename: String,
+    transform: (Int) -> T
+  ): Deferred<T> {
+    return CoroutineScope(coroutineDispatcher).async {
+      val imageId = ImageId(imageContainerType, entityId, imageType, filename)
+      val length = imageLengths.getOrPut(imageId) {
+        gcsService.fetchImageContentLengthAsync(
+          imageContainerType, imageType, entityId, filename
+        ).await() ?: -1
+      }
+      return@async transform(length.toInt())
+    }
+  }
+
+  fun retrieveImageContentAsync(
+    imageContainerType: GcsService.ImageContainerType,
+    imageType: GcsService.ImageType,
+    entityId: String,
+    filename: String
+  ): Deferred<ByteArray?> =
+    gcsService.fetchImageContentDataAsync(imageContainerType, imageType, entityId, filename)
+
+  fun computeImageUrl(
+    imageContainerType: GcsService.ImageContainerType,
+    imageType: GcsService.ImageType,
+    entityId: String,
+    filename: String
+  ): String = gcsService.computeImageUrl(imageContainerType, imageType, entityId, filename)
+
+  private data class ImageId(
+    val imageContainerType: GcsService.ImageContainerType,
+    val entityId: String,
+    val imageType: GcsService.ImageType,
+    val filename: String
+  )
+}