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/services.cpp b/adb/services.cpp
index cf346ba..335ffc4 100644
--- a/adb/services.cpp
+++ b/adb/services.cpp
@@ -227,6 +227,8 @@
             sinfo->state = kCsDevice;
         } else if (name == "-recovery") {
             sinfo->state = kCsRecovery;
+        } else if (name == "-rescue") {
+            sinfo->state = kCsRescue;
         } else if (name == "-sideload") {
             sinfo->state = kCsSideload;
         } else if (name == "-bootloader") {