Add option for switching between UMS and MTP/PTP mode. (1/2)

This adds support for switching between UMS and MTP/PTP.
Framework part (this part) will only enable UMS if the config setting is set.
Standard is to enable UMS if the primary volume supports UMS.

It's only porting this http://review.cyanogenmod.org/#/c/21117/

	modified:   services/java/com/android/server/usb/LegacyUsbDeviceManager.java
	modified:   services/java/com/android/server/usb/UsbDeviceManager.java

https://github.com/CyanogenMod/android_frameworks_base/commit/09764a4642c9ad89e4a53381bc79ae6b34ccce04

Change-Id: I2b7948895c3e0d43c2bb51df7d8b335e475d395d
Merge branch 'kitkat' of https://github.com/J-Team/platform_frameworks_base into kitkat