Command to move private app data between volumes.

New "mvuserdata" command will move all private app data from one
volume UUID to another.  It leverages the existing "cp" toybox
command to do the heavy lifting for all known users, preserving
details like timestamps and permissions.  It invokes restorecon()
to correctly label the new location when the copy is finished.

Changes installd to no longer drop capabilities, so we run as root
again.  This also allows us to exec "cp" with CAP_DAC_OVERRIDE and
CAP_FOWNER still in effect.

Bug: 19993667
Change-Id: I1f407a7c4a1af97ca5afc27b04eb16b4936cbdef