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