resourcemanager: protect resources from concurrent access

Since getClient access resources (map), make sure its protected
from the concurrent access.

Bug: 289097671
Test: atest android.media.misc.cts.ResourceManagerTest
      atest android.media.misc.cts.ResourceManagerMultiTest
      /data/nativetest64/ResourceManagerService_test/ResourceManagerService_test
      /data/nativetest64/ResourceObserverService_test/ResourceObserverService_test
Change-Id: Ica03c3dc45b97502414c76c10057acc9c9b0c03d