frameworks/base: Add 'allowMTP' option for storage volume

Added 'allowMTP' option for each storage volume to decide whether
to show up volume in MTP mode or not. If it not defined, by default
it will be visible on MTP. If someone wants to hide the volume,
one can set it 'false'.

Change-Id: I700c6c04f67f93d881aa0a4581e2d23112453f39