Add option for switching between UMS and MTP/PTP mode. (1/2)

Change-Id: I8e692d468cb36ccc97ca69b8a2eb11afeb711eae

This adds support for switching between UMS and MTP/PTP.
Framework part (this part) will only enable UMS if the config setting is set.
Standard is to enable UMS if the primary volume supports UMS.

It's only porting this http://review.cyanogenmod.org/#/c/21117/
Add UsbDebuggingManager and UsbDebuggingActivity to LegacyUsbManager

The UsbDebuggingManager listens to adbd requests and displays a dialog
when the public key authentification fails, for the user to confirm if it
wants to allow USB debugging from the attached host. If the user chooses
to always allow USB debugging, the UsbDebuggingManager writes the public
key to adbd's config file so that the public key authenfication succeeds
next time.

Change-Id: I819b8a0428ba34b68dadeb9fa67c53abb9a85fbc