Show the USB mass storage UI as soon as USB is connected.

This is a huge usability win for average users who are
trying to transfer files: it coaches them through the
process while at the same time saving a number of steps.

If the user is just charging via USB, they're most likely
not actively using the device, in which case they won't
notice the difference.

Users with ADB turned on will see the old behavior (just the
ongoing notification, which pops the UsbStorageActivity if
tapped).

Bug: 2299129

Change-Id: Ib63a0a973dfdeb28741268fcf0e0fbc1b7be2922