adb: Recognize rescue mode.

This CL adds client support to recognize the rescue mode (which will be
served by recovery image). It also allows waiting for a device to enter
rescue mode. The support for the actual rescue commands will be added in
follow-up CLs.

Bug: 128415917
Test: `adb devices` recognizes devices under rescue mode.
Test: `adb wait-for-rescue` waits for device to be in rescue mode.
Change-Id: I367d7339fe68006aba09a1e3db6370d472296676
diff --git a/adb/transport.cpp b/adb/transport.cpp
index 15c3a9a..841865a 100644
--- a/adb/transport.cpp
+++ b/adb/transport.cpp
@@ -1012,6 +1012,8 @@
             return "host";
         case kCsRecovery:
             return "recovery";
+        case kCsRescue:
+            return "rescue";
         case kCsNoPerm:
             return UsbNoPermissionsShortHelpText();
         case kCsSideload: