[incremental] create /data/incremental in init.rc

We use /data/incremental to keep all the directories that are mounted on
the Incremental File System.

Since system_server does not have permission to dynamically create dirs
directly under /data, we create /data/incremental in init.rc.

Test: boots
BUG: 136132412
Change-Id: Ic90cc8f652672a8d4459c0cd38db9c0872217af4

rootdir/init.rc

@@ -666,6 +666,9 @@ on post-fs-data
     mkdir /data/rollback 0700 system system encryption=DeleteIfNecessary
     mkdir /data/rollback-observer 0700 system system encryption=DeleteIfNecessary
 
+    # Create root dir for Incremental Service
+    mkdir /data/incremental 0770 system system encryption=None
+
     # Wait for apexd to finish activating APEXes before starting more processes.
     wait_for_prop apexd.status ready
     parse_apex_configs