Add tests for framework-specific Error Prone.

We recently started writing custom Error Prone checkers, but it's
been painfully slow to develop against the giant source tree, so
this change adds tests to verify existing behavior and to enable TDD
for future checkers.

Bug: 155703208
Test: atest error_prone_android_framework_test
Change-Id: I7ea7484db5d19e812354703e561a499077329098