remove client token passing active VI service

Previous implementation relied on client to pass a token which the
service used to verify if it was the active service. This is seen to be
a security concern as there is no way to verify how the client obtained
the token. Instead, a check is done to confirm the caller's UID matches
the UID of the active service.

In the case of voice model enrollment, KeyphraseEnrollmentInfo class is
leveraged. A client is allowed to enroll if it is the active voice
interaction service or if it is a voice model enrollment application
bundled with the system image.

All previous manifest permision checks still apply.

Bug: 148159858
Test: gts-tradefed run gts-dev -m GtsAssistIntentTestCases -t \
com.google.android.assist.gts.KeyphraseModelManagerTest \
\#testShouldEnrollOnlyWhenActiveService

Change-Id: Ie2c4653d365770a9123a22bc69822518b4ccc568