Bug 961986 - [fugu][buri] it took long time to resume music play after ending a phone call

When a call starts, the KeypadManager increases the priority of the TonePlayer's audio channel from "normal" to "telephony" so that users can hear tones when they tap the keypad during a call. We need to reset the channel back to "normal" when hanging up so that other apps can resume playing audio.

This is a minimal fix to the issue; it might make sense to have KeypadManager have a different function for resetting the channel back to "normal", since KeypadManager.init() does quite a bit of other stuff. Nevertheless, this works.

Not sure if we need tests or where they'd go... let me know if you think this should have some tests.