Update Gradle build and subsequent configuration for Android 7

- Add Google's Maven repository required by the newer Gradle for
  Android plugin version;
- Update Gradle wrapper (and include the source to benefit from the
  IDE integration) with:
  $ ./gradlew wrapper --gradle-version 4.4 --distribution-type all
- Add a lint hint to discard the warning about the target SDK version
  not being the latest one

Change-Id: I39bf8388f7b2236c92458cbf92d998321e79b3fe
6 files changed