Minimize the number of default enabled IMEs part 1

Basically this CL does following clean-ups as groundwork.
- Embed isDefaultEnabledIme into its only one caller
- Fix a typo in isSystemAuxilialyImeThatHashAutomaticSubtype()
- Use exit-early style in getMostApplicableDefaultIME()

No behavior change is intended by this CL.

BUG: 17347871
Change-Id: I831502db502f4073c9c2f50ce7705a4e45e2e1e3