[pm] fix crash caused by race condition onUidGone

GentleUpdateHelper monitors uids when they are no longer running and
informs installers when an installation can be proceeded. A race race
condition can happen when HotwordDetectionService stops running, and its
isolated uid's owner uid has also been recently removed, which causes an
uncaught exception in pm's code, causing system server crash.

BUG: 286903116
Test: builds
Change-Id: I05662d2268d6e94132fb62e75908525dccba9ecb