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