Multi-user MTP.

The current MTP kernel driver at /dev/mtp_usb is exclusive, meaning
only one process can have it open. In addition, each MTP session
with a desktop requires unique object IDs, which doesn't hold true
across users on the device.

To solve these two issues, when switching users we cycle the USB host
stack to disconnect both local and remote MTP connections, giving the
new user's media process a chance to claim /dev/mtp_usb, and causing
the desktop to initiate a new MTP session.

This change also allows BroadcastReceivers to registerReceiver()
allow retrieval of a current sticky broadcast. Adds a system property
to override maximum users. Removes MOUNTED broadcasts for secondary
users. Allows INTERACT_ACROSS_USERS to getCurrentUser().

Bug: 6925114
Change-Id: I02b4a1b535af95fb2142655887b6d15a8068d18a