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