`gclient sync` now grabs Robolectric, JUnit, and their transitive
runtime dependencies. They're being stored in a new CIPD package,
`flutter/android/robolectric_bundle`.

`shell/platform/android/BUILD.gn` has a new target for building the
tests, `robolectric_tests`. `testing/run_tests.py` has been extended to
build and run the new target. Runs the android tests under
"build_and_test_android" on CI.

This also adds some very simple sample tests to start with and a README
to the java tests directory.