Added SystemServerInitThreadPool

System services can use it during the boot to submit tasks that can be run in
parallel with the main thread.

Switched PersistentDataBlockService and FingerprintService from FgThread to
the new thread pool.

UiModeManagerService: update initial configurations on init thread. They run
while holding the mLock so no extra synchronization barriers are needed at a
later stage.

Test: manual - device boots without errors
Test: ParallelPackageParserTest passes
Change-Id: I548f34b0a18f61924e09a39afb12e085cde35442