Flush FastCapture PipeReader when starting after stop

Discards stale frames.

Bug: 30199985
Change-Id: Ie93a3784bf052aba6989d2ff1be92b1980b0c207
diff --git a/include/media/nbaio/PipeReader.h b/include/media/nbaio/PipeReader.h
index 7c733ad..00c2b3c 100644
--- a/include/media/nbaio/PipeReader.h
+++ b/include/media/nbaio/PipeReader.h
@@ -47,6 +47,8 @@
 
     virtual ssize_t read(void *buffer, size_t count);
 
+    virtual ssize_t flush();
+
     // NBAIO_Source end
 
 #if 0   // until necessary