Upgrade targetSDK to API 28 and support libraries to 28.0.0

[Daniel-Wang]

Features

• Upgrades the target SDK to API 28 (Pie)
• Upgrades support library to its final version 28.0.0. The future will be on AndroidX
• Upgrade constraint layout version to 1.3.3

[Daniel-Wang]

@MeghaBambra Just an update on my trials and successes 😛
Was able to fix a bug that gave me a heart attack, the error looked very similar to the API 19 incompatible algorithm bug. But after carefully googling and stackoverflowing, I found this issue here. And now it works on API 19+. Probably going to file a PR for the README of soneso because of this.
Also, I can't test horizon stuff because wifi is not available on emulators until API 25. But the UI components seem to not crash, which is what I was worried about for this upgrade.

[dabitdev]

Hey Daniel I am getting

Execution failed for task ':app:mergeDebugResources'.
> java.util.concurrent.ExecutionException: com.android.builder.internal.aapt.v2.Aapt2Exception: Android resource compilation failed
  Output:  /Users/David/Projects/stellar-android-wallet/app/build/intermediates/incremental/mergeDebugResources/merged.dir/values/values.xml:1269: error: duplicate value for resource 'attr/strokeColor' with config ''.
  /Users/David/Projects/stellar-android-wallet/app/build/intermediates/incremental/mergeDebugResources/merged.dir/values/values.xml:1269: error: resource previously defined here.

Did you experience any similar issue?