Enable batch mode benchmarking on iree-android-benchmark (#5106)
Enable batch mode benchmarking on iree-android-benchmark (#5106)

This produces more stable numbers because it can amortize overhead in batch mode.

Due to issues on Pixel 4 GPU, we can't let it run more than two seconds. Otherwise it will get killed. Thus, the batch mode is not enabled for Pixel 4 GPU.

## Pixel 4

MobileBert - CPU: ~660 ms
MobileBert - GPU: ~952 ms (unchanged)
MobileNetV2 - CPU: ~380 ms
MobileNetV2 - GPU: ~1024 ms (unchanged)

## S20

MobileBert - CPU: ~750 ms
MobileBert - GPU: ~270 ms 
MobileNetV2 - CPU: ~266 ms
MobileNetV2 - GPU:  ~55 ms
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