Add progress indicator to USB mass storage screen.

When the user taps on the enable button, it's hidden and
replaced with an indeterminate ProgressBar; this is in turn
hidden and replaced with the disable button in response to
broadcasts from the MountService. (Disabling UMS is
analogous.)

(Requires change Ic53cb370, which moves MountService's
operations out of the UI thread.)

Bug: 2483340
Change-Id: Ia25d99cd9709ee5a05316e8f05f66ca9da20aa25