Synchronized setClientProxy and callbackFinished to prevent NPE

During the gap between createClient and setClientProxy that sets up
mClientProxy, another thread callbackFinished on mClientProxy that will
causes NPE, this CL wait until mClientProxy is created then allow
callbackFinished to be called

Test: Added Thread.sleep(7000) to expand the gap between
mService.createClient and client.setClientProxy to expand the chance of creating race condition NPE then run PtsChreTestCases and passed (ab/I55000010077218265)
Fixes: 240691175
Change-Id: I8a729abf9f4368eab51c886a20035ccf879d9eff