Enrich test cases when enabling default IMEs part 4

This is another groundwork for subsequent fixes.

Notable changes in test cases are:
- simplified test cases for voice IMEs.
- added more test cases for keyboard IMEs.
- introduced assertDefaultEnabledImes() to reduce code duplicate.

BUG: 17347871
Change-Id: I8cf61cfa18ac425a2cccc7823474c3f06dc0fa1e