Add API for querying if a given keyphrase/locale is supported for AOHD.

Test: Use test VoiceInteraction app to verify function behaves as
expected, and no longer crashes in the framework if you try to use a
AOHD with a null KeyphraseEnrollmentMetaData object.

Change-Id: I12923b8ebf37e6de9be336b4397c24679c86fff8