Bug: 21589105 Rescoping the SYSTEM_ALERT_WINDOW permission to an explicit toggle

to be manually enabled in Settings.

Raised the protection level of SYSTEM_ALERT_WINDOW from dangerous to
system|signature|appop. Added a new API in Settings for developers to invoke
the main configuration setting. Also added a new metrics in MetricsLogger.
Finally, also made changes to PhoneWindowManager to check the permission to draw
overlay properly.

Change-Id: I4a073e6f038b8b8d2fa5bd6ad60abda496be9701