UsbModeChooserActivity: Ensure MIDI available with DISALLOW_USB_FILE_TRANSFER

In ec323a08, DISALLOW_USB_FILE_TRANSFER
turned off all USB options except for "Charge Only". This is too
aggressive. MIDI should be available when DISALLOW_USB_FILE_TRANSFER
is set.

Remove mUsbManager.isUsbDataUnlocked(). That data is available
by accessing the sticky broadcast.

Bug: 22447614
Bug: 21429947
Change-Id: Icd8398c4cf43f807acf77923b0b25c18f9a9b0d0