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
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
minivold: adjustments for vold in recovery

- Mount directly to /storage/ instead of /mnt/media_rw
- Do not attempt to mount ASEC

Change-Id: Icb088a413e07222b302264d10f7c7f2aadfebcab