TIF: getClientUserId API

Partner requested this api as System SDK API so that singleuser-ed TIS session
in owner process can get the rating settings corresponding to the TvApp's userId from TIF.

Bug: b/288383796
API-Coverage-Bug: b/374340125
Test: run cts -t android.media.tv.cts.TvInputManagerTest#testGetClientUserId
Flag: android.media.tv.flags.kids_mode_tvdb_sharing
Change-Id: I89ac347019e35a9930eb42ed324d0330b87d6540