Parse BCB command to enter rescue mode.

bootloader will set `boot-rescue` in BCB command field to indicate
booting into rescue mode. This CL adds the matching parsing code.

This CL changes the on-screen UI to display the default image while
waiting for each sideload / rescue command.

It also changes the minadbd reboot handlers to use REBOOT_ instead of
the previous ENTER_ actions. This ensures a reboot going through
bootloader, which may load a newly installed bootloader/recovery.

Bug: 128505466
Test: Boot into rescue mode. Run `adb rescue getprop` and `adb rescue
      install`. Check the UI. Then run `adb reboot rescue`.
Change-Id: I5b7de9dfd898ed8e14bea0d4ad7385a9bae26e94