Monitor device storage directly in /data/data

The DeviceStorageMonitorService watches over /data, and when things get
full, starts deleting app cached files.  The app caches are actually
under /data/data, which on DINC is its own mount.  This means the
monitor did not notice when app cache grew too big, as it only saw the
size of /data proper.

This patch makes the monitor look at /data/data instead.  For DINC, this
means it can properly manage app cache, but it won't notice if the upper
/data partition is getting full.  That's not as common as /data/data
filling up though.  For the Samsung devices that mount a /datadata and
then symlink to /data/data, it should also work, as statfs dereferences
links first.  For all other devices with only /data, reading statfs on
/data/data still yields the total/free sizes of the /data partition.

Change-Id: I118a85a04c3a257d54d40ce7802426b4e0fe3e87