Camera: Remove redundant RequestThread wait during flush

If RequestThread::clear() is called right before the
mRequestSignal.waitRelative(), an extra 50ms latency is incurred.

Creating a condition for the conditional variable fixes the issue.

Test: Camera CTS
Bug: 228419719
Change-Id: Ibc8d2c9e2c98d68b73d2442118694c7a33e9d3b5
diff --git a/services/camera/libcameraservice/device3/Camera3Device.h b/services/camera/libcameraservice/device3/Camera3Device.h
index 1a240c3..749b342 100644
--- a/services/camera/libcameraservice/device3/Camera3Device.h
+++ b/services/camera/libcameraservice/device3/Camera3Device.h
@@ -982,6 +982,8 @@
 
         Mutex              mRequestLock;
         Condition          mRequestSignal;
+        bool               mRequestClearing;
+
         Condition          mRequestSubmittedSignal;
         RequestList        mRequestQueue;
         RequestList        mRepeatingRequests;