rtc alarm: fix bad index when canceling alarms[]

(backport from common android-3.0
commit: 692e468137ebb2a31431c4b3fad1dca0a2da7659)

It was using ANDROID_ALARM_ELAPSED_REALTIME_WAKEUP_MASK as an
index.

Change-Id: I919860cc71254453e382616bce9fd5455802cb3d
Signed-off-by: JP Abgrall <jpa@google.com>