adb remount -R should enable DSU if suitable

If executing `adb remount -R` and DSU is running but disabled, then
enable the DSU (one-shot mode) so that the reboot afterwards would stay
within the DSU guest system.
Normally reboot within a DSU guest system would bring the device back to
the host system. However when doing adb remount -R, we actually doesn't
want to exit DSU, but wish to reboot back into DSU guest system again
with remount machinery (overlayfs) properly set up.

Also sort the header include order.

Bug: 165925766
Test: Within a DSU guest system, DSU disabled, adb remount -R
  => After reboot, system is DSU and overlayfs is mounted
Test: adb-remount-test.sh within DSU guest system
Change-Id: I72a7a568e985b183d357ae6e1a7d0113e9921200