Permission check for entering MODE_IN_CALL

Make the ability to change the audio mode conditional to having
 the MODIFY_PHONE_STATE permission.
Make AudioService's internal implementation of the mode setting
 (the setModeInt() method) private, it is only meant to be used
 by AudioService.
Refer to the permission names through Manifest.permission, not
 by their string value.

Bug 14964198

Change-Id: I49870207566e53d5c8764646cece06d8272f93ed