Update app_api to fix style/lint issues

Test: manually invoked/profiled MainActivity

- Added Nullable/NonNull annotations
- Added RequiredAPI annotations
- Switch to consistent mFoo naming

And several other lint suggestions Android Studio suggested.

Additionally, updated target/compile API of demo.

These lint / style issues were found when copying this code into
Jetpack Benchmark, in the AndroidX repository:

https://android-review.googlesource.com/c/platform/frameworks/support/+/1343923

Change-Id: I9fd876f1d15a4812d1ef52958661050b258ca38d
3 files changed