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