Introduce a simulated GAE proto endpoint.

This 'simulates' the eventual endpoint that will be added to Oppia web
by using internal compatibility checking, proto conversion, and the
GAE/GCS JSON endpoints currently available via Oppia web.

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

@@ -0,0 +1,36 @@
+"""
+Library for providing access to Oppia's HTTP endpoints.
+"""
+
+load("@io_bazel_rules_kotlin//kotlin:jvm.bzl", "kt_jvm_library")
+
+kt_jvm_library(
+    name = "gae",
+    testonly = True,
+    srcs = [
+        "GaeAndroidEndpoint.kt",
+    ],
+    visibility = ["//scripts:oppia_script_library_visibility"],
+    deps = [
+        "//third_party:oppia_proto_api_java_protos",
+        "//third_party:org_jetbrains_kotlinx_kotlinx-coroutines-core",
+    ],
+)
+
+kt_jvm_library(
+    name = "gae_json_impl",
+    testonly = True,
+    srcs = [
+        "GaeAndroidEndpointJsonImpl.kt",
+    ],
+    visibility = ["//scripts:oppia_script_library_visibility"],
+    deps = [
+        ":gae",
+        "//scripts/src/java/org/oppia/android/scripts/gae/compat",
+        "//scripts/src/java/org/oppia/android/scripts/gae/json:api",
+        "//scripts/src/java/org/oppia/android/scripts/gae/json:model",
+        "//scripts/src/java/org/oppia/android/scripts/gae/proto:json_to_proto_converter",
+        "//third_party:oppia_proto_api_java_protos",
+        "//third_party:org_jetbrains_kotlinx_kotlinx-coroutines-core",
+    ],
+)

scripts/src/java/org/oppia/android/scripts/gae/GaeAndroidEndpoint.kt

@@ -0,0 +1,19 @@
+package org.oppia.android.scripts.gae
+
+import kotlinx.coroutines.Deferred
+import org.oppia.proto.v1.api.TopicContentRequestDto
+import org.oppia.proto.v1.api.TopicContentResponseDto
+import org.oppia.proto.v1.api.TopicListRequestDto
+import org.oppia.proto.v1.api.TopicListResponseDto
+
+interface GaeAndroidEndpoint {
+  fun fetchTopicListAsync(
+    request: TopicListRequestDto,
+    reportProgress: (Int, Int) -> Unit = { _, _ -> }
+  ): Deferred<TopicListResponseDto>
+
+  fun fetchTopicContentAsync(
+    request: TopicContentRequestDto,
+    reportProgress: (Int, Int) -> Unit = { _, _ -> }
+  ): Deferred<TopicContentResponseDto>
+}