Settings: fix storage measurement for device without emulated sdcard

StorageMeasurement: misc files should only be measured for internal
storage only if it is the emulated storage, otherwise misc count
for the primary storage will be erroneously duplicated on the
internal storage count.

StorageVolumePreferenceCategory:
  - Always create mMountTogglePreference to avoid null pointer
    exception for code that relies on it existing
  - Check if the keys exist before calculating totalValues to
    avoid null pointer exception on storage where media is not
    calculated
  - For available/readonly, set title, not summary (summary is
    supposed to be the amount available)

Change-Id: Ib217f79ee562dffc514696fff038e58ed0dba7d9
Add option for switching between UMS and MTP/PTP mode. (2/2)

Change-Id: I6cf50a41a9a82e40e702f5231ce663636c149613

This adds support for switching between UMS and MTP/PTP.
Settings part (this part) adds option in USB settings (statusbar/notification drawer) for enabling UMS,
and adds setting in Storage menu to enable / disable UMS, this since the USB settings page disappears when enabling UMS.

It's only porting this http://review.cyanogenmod.org/#/c/21115/

Conflicts:
	res/values/strings.xml