UsbSettings: Add UMS mode support.

Add Usb Mass storage mode support for Android device;
Fix SD card can not be mounted to device when usb cable
is plugged in or unplugged fast and frequently.

Change-Id: Iff6de6b1b3e066f4a4ab610ae3fe10b22f5b0822
UsbSettings: Adjust UMS mode code logic.

Not to update UMS status in every iteration of client listener.

Change-Id: I5fbc0145c798c468f2ae943b251700f1a71e00d8