SoftVPX related perf tuning

- Boost decoding thread priority to match ACodec's for video

- Bump up SoftVPX input/output buffer count to 16

- Set buffer count in native window when SoftwareRenderer is used

- Set native window usage to READ_NEVER | WRITE_RARELY to avoid
  unnecessary cache invalidation upon lock/unlock

bug: 72841545
Change-Id: Ib7ac3f881fd0612884ecee68ffc79d6fa8f8ca37