Fix the issue of NV12 input with wrong format VPX_IMG_FMT_I420

C2SoftVpxEnc uses NV12 input data but format is specified as I420,
which causes the recorded video messy.

The fix is using mConversionBuffer as input, whose format is I420.

Bug: 139906732
Test:
Change-Id: Ib94dd217a82b2bac57056cfc3af5344da78b47d7
1 file changed