Wait for idmap service on current thread

IdmapDameon should wait for idmap service to be brought up rather than
schedule a task on a different thread to retrieve the service.
Regardless, the current thread will need to block a certain amount of
time until the idmap service is posted after idmap2d starts.

Bug: 142542849
Test: atest OverlayHostTests
Test: manually enabling and disabling overlays
Change-Id: I36810ffa3a690b52bbe77845aa1f55a20f090f82