Camera2: Add flush support

- On HAL2 devices, fall back to wait until idle
- On HAL3 devices, call HAL flush method

Bug: 9758581
Change-Id: Ie1c570a15f6590a1ee6c271e3b989c48079b468a
diff --git a/services/camera/libcameraservice/api2/CameraDeviceClient.h b/services/camera/libcameraservice/api2/CameraDeviceClient.h
index 21d633c..c6b6336 100644
--- a/services/camera/libcameraservice/api2/CameraDeviceClient.h
+++ b/services/camera/libcameraservice/api2/CameraDeviceClient.h
@@ -89,6 +89,10 @@
 
     // Wait until all the submitted requests have finished processing
     virtual status_t      waitUntilIdle();
+
+    // Flush all active and pending requests as fast as possible
+    virtual status_t      flush();
+
     /**
      * Interface used by CameraService
      */