Fix NullPointerException when using vibrate-only alarms.

mAudioManager wasn't initialized in that case.

Fixes issue CYAN-376.

Change-Id: I70b3bc1f89498c3407980f512b09c3449969e733
Merge "Fix NullPointerException when using vibrate-only alarms." into cm-10.1
Merge branch 'cm-10.1' of git://github.com/CyanogenMod/android_packages_apps_DeskClock into cm-10.1