adb: tell the client what transport it received.

Prerequisite for making `adb root` wait for the device that it told to
restart to disappear: the client needs to know which transport to wait
on.

Bug: http://b/124244488
Test: manual
Change-Id: I474559838ad7c0e961e9d2a98c902bca3b60d6c8
diff --git a/adb/adb.h b/adb/adb.h
index f575adb..5eea8be 100644
--- a/adb/adb.h
+++ b/adb/adb.h
@@ -219,8 +219,15 @@
 #define USB_FFS_ADB_IN USB_FFS_ADB_EP(ep2)
 #endif
 
-bool handle_host_request(std::string_view service, TransportType type, const char* serial,
-                         TransportId transport_id, int reply_fd, asocket* s);
+enum class HostRequestResult {
+    Handled,
+    SwitchedTransport,
+    Unhandled,
+};
+
+HostRequestResult handle_host_request(std::string_view service, TransportType type,
+                                      const char* serial, TransportId transport_id, int reply_fd,
+                                      asocket* s);
 
 void handle_online(atransport* t);
 void handle_offline(atransport* t);