Allow automated sideload from within recovery.

This uses the recovery socket to restart in sideload mode without a full
reboot. This will greatly enhance our ability to test sideload behavior
as we won't need an intermediate reboot.

The switch to sideload can be triggered with:

    adb root
    adb shell "echo -n 's' | nc -U /dev/socket/recovery"

Bug: 416748567
Test: manual test
Change-Id: Ie2c1bb61aa04914a143c1e860b0c5cd475c10148
Signed-off-by: David Anderson <dvander@google.com>