Reland "Add microdroid_vendor"

This reverts commit c79d1dc7ebd40d182816ecb4baaa1c0b6afa8b2a.

Reason for revert: relanding

Change-Id: I38b51ef26e62900f5f14230b1c08681ea7871d13
diff --git a/microdroid/fstab b/microdroid/fstab
index 494f5b4..129718e 100644
--- a/microdroid/fstab
+++ b/microdroid/fstab
@@ -1 +1,2 @@
 system /system ext4 noatime,ro,errors=panic wait,first_stage_mount,logical
+vendor /vendor ext4 noatime,ro,errors=panic wait,first_stage_mount,logical