Set GID required to write, media_rw mount point.

Add sdcard FUSE daemon flag to specify the GID required for a package
to have write access.  Normally sdcard_rw, but it will be media_rw
for secondary external storage devices, so DefaultContainerService
can still clean up package directories after uninstall.

Create /mnt/media_rw which is where vold will mount raw secondary
external storage devices before wrapping them in a FUSE instance.

Bug: 10330128, 10330229
Change-Id: I4385c36fd9035cdf56892aaf7b36ef4b81f4418a