Fix confusing warning dialog behaviour.

The settings app closing when pressing back while the dialog is shown or
clicking outside of the dialog was confusing for the user. Instead, make
sure the user has to acknowledge the dialog.

Change-Id: I8ffdf6a2ede8b67176fd22a13d7046a707b52b89

Conflicts:
	src/com/android/settings/cyanogenmod/PerformanceSettings.java