DeskClock: Handle invalid default alarm URI

If the user sets an external file as their alarm and the file is
subsequently deleted, DeskClock will go into a crash loop trying
to reset to a default alarm uri (which was set to this external
file).

This patch will recover when this scenario is triggered, and the
default uri is invalid. DeskClock will fetch and reset default alarm
URI from the system property 'ro.config.alarm_alert'. If no such system
property exists or the file cannot be found, it will pick a random
alarm tone from MediaStore.

Change-Id: I481876e0a25fbbb7ab1be8e3d7c9f8a94d66a43c