Move API lint stubs to common class

Additionally,

 - Move linters to new directory to match package name. This allows the
   tests to be run at the package level in Android Studio.

 - Use @Suppress("UnstableApiUsage") for all linter classes

 - Add missing copyright headers

Test: manual
Bug: 238923086
Change-Id: I1ee616a34e6e16586847c3a68f751efe40d7f565