[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