Mark block device as rw before encryptFstab

Some testcases assume that /dev/block/by-name/userdata is writable, but
mount_with_alternatives() will mark block device as RO if mount flag
includes MS_RDONLY. Fix it by marking the block device as RW again.

Test: th
Bug: 319156415
Change-Id: Ic04acd4b6175d3f0aeea88675da44309e8df15e8