FileUtils: add method to get UUID of a volume

Change-Id: I41b76c5e01c82a2f8e67b33133a76947877a2814

Conflicts:
	core/java/android/os/FileUtils.java
	core/jni/Android.mk
	core/jni/android_os_FileUtils.cpp
Put FileUtils back into android_runtime

We still use it for non-fat volume management

Change-Id: I42efeb83b8f12e7734e4bf03326526cc220d022a