Don't call ActivityManagerService on main thread

AttentionManagerService is getting called from PMS with its mLock held.
PowerManagerService can't up-call to the system, thus AMS can't call
into ActivityManagerService via bindService() - put it on another
thread.

Bug: 133077390
Test: manually
Change-Id: I373163c188bc8dbbd3d4f708073a875a57cfed73