Fix Android Test build errors caused by Gutenberg change

[jtreanor]

The changes from #9644 are causing build errors for our connected tests. You can see the failure on CircleCI here.

The problem is that the new compileOptions entry
isn't playing nicely with the Android tests. I'm not sure why this is. Since the setting is only needed when building from source, I have added a conditional check around it.

Note: assembleVanillaDebugAndroidTest will still fail when building Gutenberg from source. I tried moving the Gradle config to WordPressEditor but that didn't work.

To test:

With wp.BUILD_GUTENBERG_FROM_SOURCE=false:

• ./gradlew WordPress:assembleVanillaDebug
• ./gradlew WordPress:assembleVanillaDebugAndroidTest

With wp.BUILD_GUTENBERG_FROM_SOURCE=true:

• ./gradlew WordPress:assembleVanillaDebug

Update release notes:

• If there are user facing changes, I have added an item to RELEASE-NOTES.txt.

[mzorz]

LGTM!