Fix issue where keyguard adds widgets before the system is ready

While under heavy system load,  keyguard was able to create widgets before
before ActivityManagerService was ready.  The result was a race
between keyguard adding widgets and ActivityManagerService being
ready to send broadcasts.

This fix provides keyguard with an additional signal to know when
the system is booted and widgets are safe to load.

Fixes bug b/11217169

Change-Id: I7a714d65b068678f961e52bdde4e1c20f9c287f0