Add UID check when starting MediaProjection sessions

Prevent MediaProjection tokens from being exchanged between
applications.

Also fix the unit tests that effectively relied on this behavior.

Bug: 419269649
Test: atest MediaProjectionManagerServiceTest
Flag: com.android.media.projection.flags.start_uid_check
Change-Id: I7e2c74220171484338aa339387e484b2794c9c65