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
diff --git a/bootstat/bootstat.cpp b/bootstat/bootstat.cpp
index 418c711..a0a9307 100644
--- a/bootstat/bootstat.cpp
+++ b/bootstat/bootstat.cpp
@@ -202,6 +202,8 @@
   {"reboot,ota", 63},
   {"reboot,factory_reset", 64},
   {"reboot,", 65},
+  {"reboot,shell", 66},
+  {"reboot,adb", 67},
 };
 
 // Converts a string value representing the reason the system booted to an