Disable GlowPad animation when alert activity is done

Bug: 9825266
The mPingEnabled was not set to flase when the alarm was dismissed, so the
handler was restarting the ping animation of the GlowPad.

Change-Id: I8e0de4ecd86a2a4df7237cd4cbe71aa97272a089
(cherry picked from commit b18aea79)