Make clean_scratch_files work

clean_scratch_files.rc seems to never work as intended due to
missing domain transition rules.
Add a 'remount' domain to the platform policies to allow
remount-related operations, including clean_scratch_files.
Merge remount and clean_scratch_files binary to simplify the added
policies.

Bug: 204836146
Test: 1. Use a VAB device, for example bramble-userdebug
  2. adb remount system
  3. adb reboot fastboot && fastboot flash system system.img
  4. recovery (fastbootd) should mark the remount scratch as disabled
  5. fastboot reboot && adb shell mount | grep scratch => nothing
  6. adb logcat | grep 'clean_scratch_files|gsid' => shows that
    disabled image "scratch" is removed during boot
Change-Id: I18d543868d1f37d43b483eae7517b707e46df1bd