vold: Add selinux support to ext4

Squashed commit of the following:

Author: Pawit Pornkitprasan <p.pawit@gmail.com>
vold: mount ext4 with forced SELinux context

Always mount voldmanaged ext4 partitions with the SELinux sdcard_external
context. For other file systems, this is done via the genfs_context,
but since ext4 supports SELinux natively, it must be set here.
Change-Id: I1364f37018b25d79b1826c85849def64e868d72f

Author: Pawit Pornkitprasan <p.pawit@gmail.com>
vold: force SELinux context only for sdcard

Google uses this code to mount ASEC as well and we shouldn't force
their SELinux contexts
Change-Id: I873af4dc2309d3d0942ce466b8acf8158abb85ae

Author: Jorge Ruesga <jorge@ruesga.com>
vold: pass selinux context to ext4 mount (remount ro)
Change-Id: Ic0bb314b30e42489c45caec29d35d6896c9849eb
Signed-off-by: Jorge Ruesga <jorge@ruesga.com>

Change-Id: I1364f37018b25d79b1826c85849def64e868d72f