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/adb.cpp b/adb/adb.cpp
index e417f05..2dd22b3 100644
--- a/adb/adb.cpp
+++ b/adb/adb.cpp
@@ -280,6 +280,9 @@
     } else if (type == "sideload") {
         D("setting connection_state to kCsSideload");
         t->SetConnectionState(kCsSideload);
+    } else if (type == "rescue") {
+        D("setting connection_state to kCsRescue");
+        t->SetConnectionState(kCsRescue);
     } else {
         D("setting connection_state to kCsHost");
         t->SetConnectionState(kCsHost);