Postpone the shared user checking after Q

The shared user checking becomes strict. If there is an APK in
vendor partition shared UID with a APK in system partition, the
signatures of these 2 APKs must be equal. Otherwise, framework
will throw an exception and cannot boot.

Postpone the checking from first_api_level 28 to 29.

Bug: 137524152
Test: test on a device with first_api_level=29 and pass the checking
Change-Id: I76d87a6dab35d4d39113cdd16a20706ce2c45093