Enrich test cases when enabling default IMEs part 2

This is another groundwork for subsequent fixes.

Notable changes in test cases are:
- isSystemReady == false is now coverted.
- tests can run even when the system locale of test
  environment is different from "en_US"

BUG: 17347871

Change-Id: I9434270735d3b37d8b788d41250b23d7950aaeed