Make avg_pool_float_2_relaxed possible to pass with fp16 accumulator.

avg_pool_float_2_relaxed is computing an average pooling with window size
~50*50. All input elements are 1, and each output is expected to be ~2500/2500 = 1.0f.
However, for FP16, 2048+1 = 2048, so the actual output would be 2048/2500.

Bug: 125494355
Test: avg_pool_float_2_relaxed with modifed reference AveragePool to emulate fp16 accumulator,
Change-Id: I90c63fce3548e7a074c738a7fd8381ab14ab0781