Clean up USB Manager and fix ADB.

Moved functions which parse the USB functions list into one common
place on UsbManager.

Deleted the no longer supported USB_FUNCTION_MASS_STORAGE.

Ensured that the UserManager.DISALLOW_USB_FILE_TRANSFER rule is
consistently applied during user switch and when changing the
current USB functions and make sure it only affects MTP and PTP.

Collapsed the boot completed and user switched receivers to
ensure consistent ordering of side-effects.

Validate the list of functions passed to setCurrentFunction() so
that the separation of concerns is clearer.  It was somewhat
ambiguous as to whether functions such as ADB could / should be
enabled through that interface.  Improved the docs for clarity.

Fixed a bunch of broken stuff related to the USB config
persistent property (list of default functions) that could cause
ADB and other functions to not work at all.  Added new failsafes
to ensure that we reliably get back into a happy state.

Bug: 22206076
Change-Id: I02915ddfce7193a8f67a14f0d76bab22fc575dfa