Indicate sync frames returned by encoder

Set the appropriate OMX flag when the encoder generates a keyframe. This
is necessary for any muxer which needs to indicate which frames are
seekable.

Bug: 8422347
Change-Id: I744a0b3023db24d3de2210bce82f41e50d259505
diff --git a/media/libstagefright/codecs/on2/enc/SoftVPXEncoder.cpp b/media/libstagefright/codecs/on2/enc/SoftVPXEncoder.cpp
index 5f2b5c8..16f0f30 100644
--- a/media/libstagefright/codecs/on2/enc/SoftVPXEncoder.cpp
+++ b/media/libstagefright/codecs/on2/enc/SoftVPXEncoder.cpp
@@ -803,6 +803,8 @@
             if (encoded_packet->kind == VPX_CODEC_CX_FRAME_PKT) {
                 outputBufferHeader->nTimeStamp = encoded_packet->data.frame.pts;
                 outputBufferHeader->nFlags = 0;
+                if (encoded_packet->data.frame.flags & VPX_FRAME_IS_KEY)
+                  outputBufferHeader->nFlags |= OMX_BUFFERFLAG_SYNCFRAME;
                 outputBufferHeader->nOffset = 0;
                 outputBufferHeader->nFilledLen = encoded_packet->data.frame.sz;
                 memcpy(outputBufferHeader->pBuffer,