MountService: Check for failure to unmount when enabling UMS and fix reversed logic
MountService: Check for failure to unmount when enabling UMS and fix reversed logic

Signed-off-by: San Mehat <b23d0c7bb708db91c8b9415ae7caad6fe333f717@google.com>

xorware/android_frameworks_base,xorware/android_frameworks_base,xorware/android_frameworks_base,xorware/android_frameworks_base,xorware/android_frameworks_base,xorware/android_frameworks_base