recovery: Provide sideload cancellation

We can't use InterruptWaitKey() as it hangs recovery
when called from the minadbd listener thread, so provide
our own 'CancelWaitKey' implementation.

[forkbomb: rework for Q]
Change-Id: I13f0c9ae5444652a2141442ef24258679a78d320
diff --git a/recovery_ui/ui.cpp b/recovery_ui/ui.cpp
index aced5b0..17f28df 100644
--- a/recovery_ui/ui.cpp
+++ b/recovery_ui/ui.cpp
@@ -554,6 +554,10 @@
   return key;
 }
 
+void RecoveryUI::CancelWaitKey() {
+  EnqueueKey(KEY_REFRESH);
+}
+
 void RecoveryUI::InterruptKey() {
   {
     std::lock_guard<std::mutex> lg(key_queue_mutex);