adb: reboot: last boot command default

For reboot [reboot_arg] requests via either reboot or adb reboot,
if reboot_arg is empty then report "shell" or "adb" respectively.

Test: boot_reason_test.sh shell_reboot adb_reboot
Bug: 63736262
Change-Id: Ie613d9e62db6a705885e4e7520aede27af3aa1b9