adbd is allowed to execute shell in recovery mode

The shell is now available directly in the recovery ramdisk. We no
longer need to mount system.img to /system as the recovery ramdisk is
self-contained. However, there is a problem that every file in the
ramdisk is labeled as rootfs because the ramdisk does not support xattr.

This CL adds several recovery-only rules that are required to make the
recovery ramdisk self-contained. Most importantly, adbd is allowed to
domain_trans to shell. Also shell is allowe to execute files of type
rootfs. Finally, the recovery is allowed to mount on tmpfs since it now
mounts system.img to /mnt/system.

Bug: 63673171
Test: `adb reboot recovery; adb devices` shows the device ID
Test: `adb root && adb shell` and then
$ lsof -p `pidof adbd` shows that libm.so, libc.so, etc. are loaded from
the /lib directory.

Change-Id: If21b069aee63541344a5ca8939fb9a46ffef4d3e