Introduce InputMethodCoreTests

This CL extracts out all the IME related unit tests from
FrameworksCoreTests to a newly introduced InputMethodCoreTests.
This migration allows us to

 * protect IME unit tests from any unrelated test breakages because of
   unrelated tests and broader OWNERS.
 * run only IME unit tests by "atest InputMethodCoreTests".
 * have a separate presubmit rules.

Fix: 322237382
Test: atest InputMethodCoreTests
Change-Id: I5e1bdbd41d017c0acd4eaf56c00e3a7613634978