alarm: add JNI interface for clearing alarm

Add the JNI for clearing cancelled alarm, otherwise kernel driver
would know nothing about the alarm status. E.g. power-off alarm
would not be really cleared in alarm driver even it is cancelled
in the apps/framework.

Change-Id: I1250b3778de8961ba2a89d8ea8fa29d729bbc4e8

Conflicts:
	services/jni/com_android_server_AlarmManagerService.cpp