Fix race condition in MTP/PTP checkbox updating

UsbManager.getDefaultFunction() might not return the updated state immediately
after changing the USB configuration, so do not rely on it for updating
after a checkbox has been clicked.

Change-Id: I5ae8d46854f24403cda2c62e006c742dcd257d1d
Bug: 5334777

Signed-off-by: Mike Lockwood <lockwood@android.com>