Add /mnt/vendor rw mount point for vendor partitions.

Changes to init's behavior during early mount:
1. Mounting of tmpfs on /mnt is moved from init stage to early mount.
2. init creates /mnt/vendor used to mount vendor partitions.
3. If a device tree fstab entry for early mount specifies a mount point
under /mnt/vendor e.g. /mnt/vendor/foo, init will create
/mnt/vendor/foo mount point.

Bug: 64905218
Test: change dt fstab entry to mount persist to /mnt/vendor/persist;
mount point is created correctly, and partition is mounted in early
mount. See go/pag/1069774
Test: device boots with /mnt/vendor and previous contents of /mnt present,
and selinux label "mnt_vendor_file" is applied correctly.
Test: cts-tradefed run commandAndExit cts --skip-all-system-status-check
--primary-abi-only --skip-preconditions -m CtsAppSecurityHostTestCases
-t android.appsecurity.cts.PermissionsHostTest
Change-Id: I3739130739eadf508355c7f2531366fcaed74175