[Ravenwood] A few clean-ups

- Now unsupported Context API will throw with a more useful message.
- Change all the "stub"s in the policy files to "keep"s. They are the
  same thing (with the current use of hoststubgen) and "keep" matches
  the annotation name better.

Bug: 292141694
Test:  $ANDROID_BUILD_TOP/frameworks/base/ravenwood/scripts/run-ravenwood-tests.sh

Change-Id: I56b60543801ebe9a4f1ba253996edc80f1bf3e32