Do not throw RemoteException for notifyUserActivity

PowerManager does not rely on TelephonyManager#notifyUserActivity
to complete successfully since it is a oneway notification. If it
fails, due to telephony not being alive, do not throw a
RemoteException.

Bug: 156203930
Test: manual
Change-Id: I4c788fef5a887c45dbc3a20b7c8af40beed8d00a