Flush FastCapture PipeReader when starting after stop

Discards stale frames.

Bug: 30199985
Change-Id: Ie93a3784bf052aba6989d2ff1be92b1980b0c207
diff --git a/include/media/nbaio/NBAIO.h b/include/media/nbaio/NBAIO.h
index 120de4f..c22c476 100644
--- a/include/media/nbaio/NBAIO.h
+++ b/include/media/nbaio/NBAIO.h
@@ -266,6 +266,17 @@
     //              One or more frames were lost due to overrun, try again to read more recent data.
     virtual ssize_t read(void *buffer, size_t count) = 0;
 
+    // Flush data from buffer.  There is no notion of overrun as all data is dropped.
+    // Flushed frames also count towards frames read.
+    //
+    // Return value:
+    //  >= 0    Number of frames successfully flushed
+    //  < 0     status_t error occurred
+    // Errors:
+    //  NEGOTIATE         (Re-)negotiation is needed.
+    //  INVALID_OPERATION Not implemented
+    virtual ssize_t flush() { return INVALID_OPERATION; }
+
     // Transfer data from source using a series of callbacks.  More suitable for zero-fill,
     // synthesis, and non-contiguous transfers (e.g. circular buffer or readv).
     // Inputs:
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