Openmp blocktime 1->20ms to keep cores enabled

With 1ms blocktime the benchmark app shows 3x performance degradation on
Pixel 2 in float benchmarks (against no blocktime setting).

In testing, 10ms was enough to give results comparable to no setting.
20ms was chosen as to have a 2x margin of safety while not keeping the
threads busywaiting unnecessarily long.

Bug: 109645291
Test: NeuralNetworksTest_static/*Openmp*
Test: Run benchmark app multiple times
Change-Id: Id2a909f6aefdbe29dd746019c2c9541e31d8c754