add KeyphraseEnrollmentInfo to TestApi

KeyphraseEnrollmentInfo is used for testing the system enrollment
application support within GTS.

Bug: 153264697
Test: gts-tradefed run gts-dev -m GtsAssistIntentTestCases \
-t com.google.android.assist.gts.KeyphraseModelManagerTest# \
testAllSystemKeyphrasesAndLocales

Change-Id: I66558cf849f2815f2c917d128f9f9cc0673750ef