C2SoftVpxEnc: Make VPX_VBR as default mode

Use VPX_VBR as default bitrate mode instead of VPX_CBR

VideoEncoderDecoderTest in CTS doesn't set bitrate mode and
VPX_CBR mode shows encoding artifacts when running that test

Test: cts-tradefed run cts -m CtsVideoTestCases  -t \
 android.video.cts.VideoEncoderDecoderTest#testAvcGoog0Qual0640x0360
Bug: 135891350

Change-Id: I488273113b4d5cf4ae2f4b8208fcebdd095aa6f5
diff --git a/media/codec2/components/vpx/C2SoftVpxEnc.h b/media/codec2/components/vpx/C2SoftVpxEnc.h
index 90758f9..62ccd1b 100644
--- a/media/codec2/components/vpx/C2SoftVpxEnc.h
+++ b/media/codec2/components/vpx/C2SoftVpxEnc.h
@@ -275,7 +275,7 @@
         addParameter(
             DefineParam(mBitrateMode, C2_PARAMKEY_BITRATE_MODE)
                 .withDefault(new C2StreamBitrateModeTuning::output(
-                        0u, C2Config::BITRATE_CONST))
+                        0u, C2Config::BITRATE_VARIABLE))
                 .withFields({
                     C2F(mBitrateMode, value).oneOf({
                         C2Config::BITRATE_CONST, C2Config::BITRATE_VARIABLE })