Keyguard: Allow applications to customize left-corner widget

Adds ability for trusted applications to set the application widget and
action in the lower left corner of the Keyguard.  Its behavior is
modeled against the on-screen camera widget most commonly found on the
lower right corner of the Keyguard.

Three intents are defined: one to set the widget, one to clear the
widget, and one to notify when the widget is activated.

Applications calling this intent must have this permission:
android.permission.SET_KEYGUARD_APPLICATION_WIDGET.

Change-Id: I8c5b11fd1cc1888c710bf3736e3c28d77fc57e37