Start FontManagerService Async

The service used to be started on the critical boot path. Since fonts
will not be needed for a while after when the service got started, we
can offload the service-start work (i.e. creating/reading the font files
and setting font map) to a separate thread.

Bug: 291675714
Test: boot succeeds

Change-Id: Ie851e7ab5dc0e408d4ebc24bc0ebc57119464a43