transcoder benchmarks: Use static libmediatranscoder

Using static libmediatranscoder makes it easy to run the
benchmark tests without a need to push libmediatranscoder.so
to the device

Bug: 173627260
Test: atest MediaTrackTranscoderBenchmark \
      -- --enable-module-dynamic-download=true
Test: atest MediaTranscoderBenchmark \
      -- --enable-module-dynamic-download=true
Test: atest MediaSampleReaderBenchmark \
      -- --enable-module-dynamic-download=true

Change-Id: Iacad35d17a24c1a41160604b13d3f836bfce1a57