Add /metadata to ramdisk.

Now that GKI may not be writeable, also create /metadata at build time
to avoid error log at boot time.

Note that this also creates /first_stage_ramdisk/metadata in GKI.

[    1.891172] init: [libfs_mgr]check_fs():
mount(/dev/block/by-name/metadata,/metadata,ext4)=-1: No such
file or directory

Bug: 173425293
Test: boots

Change-Id: I62d23c382ed1ad165c1d6598b3df41bd92206733