Broadcast hidden volumes, notification polish.

Send limited broadcast intent when certain volume state changes
occur; the only customer for now is ExternalStorageProvider.

Change notification flow to be less bumpy.  Pick USB icon based on
disk type, and avoid using "generic" disk labels.

Bug: 19993667
Change-Id: I263bc9e9aae2ae57eb4d1afe76da686aee5475fb