Fix issue #8470131: Process thrash kills battery

Protect app widget broadcasts from abuse.

In this case the app was sending an APPWIDGET_UPDATE broadcast
without specifying a target, which (a) should not be allowed (you
should not be able to send updates to other apps), and (b) resulted
in every single potential app widget in the system being launched...
which was about 75 of them.

Change-Id: I9d48733610ce6d5a7c32e69a3e06b9f33bd79a34