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/include/camera/camera2/ICameraDeviceUser.h b/include/camera/camera2/ICameraDeviceUser.h
index 45988d0..f71f302 100644
--- a/include/camera/camera2/ICameraDeviceUser.h
+++ b/include/camera/camera2/ICameraDeviceUser.h
@@ -63,6 +63,9 @@
 
     // Wait until all the submitted requests have finished processing
     virtual status_t        waitUntilIdle() =  0;
+
+    // Flush all pending and in-progress work as quickly as possible.
+    virtual status_t        flush() = 0;
 };
 
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