Handle PHASE_BOOT_COMPLETED asynchronously in LockSettingsService

SystemService#onBootPhase(PHASE_BOOT_COMPLETED) can be called on
system_server's main thread after it has enabled StrictMode.  Therefore
I/O should not be performed.  Do the relevant work on the service thread
instead.  There is no strict timing required for the work that is being
done, so this should be fine.

Bug: 333057214
Flag: EXEMPT bugfix
Test: atest FrameworksServicesTests:com.android.server.locksettings
Change-Id: If96637fb83054b77857387415e1e6cee1c2ba7f6