minadbd: Support `adb reboot` under sideload/rescue modes.

Bug: 128415917
Test: Run the following commands under sideload and rescue modes
      respectively.
$ adb reboot
$ adb reboot bootloader
$ adb reboot recovery
$ adb reboot rescue
$ adb reboot invalid
Change-Id: I84daf63e3360b7b4a0af5e055149a4f54e10ba90