Prevent GC requests from piling up in LockSettingsService

Don't allow an unbounded number of GC requests to be enqueued if
lockscreen credential operations are happening very quickly.

Test: 'atest FrameworksServicesTests:com.android.server.locksettings'
      and checked log.  No longer see any messages that say
      "WaitForGcToComplete blocked Explicit on Explicit".  Note, even
      before this CL only a few such messages appeared with that atest
      command, but apparently it could happen more often in some cases.
Bug: 367713409
Flag: EXEMPT bugfix
Change-Id: I598116754210aa75265a9262e7b63937e2b836c2