[XLA] Increase the sample size for PrngTest.Uniformity256.

The original test samples 256 numbers with a range of 256 and performs a
ChiSquare test with 0.05 level of significance. Using the Philox RNG algorithm
on GPU, this test produces error like this:
Expected: (UniformChiSquared(256, 256)) < (293.248), actual: 300.086 vs 293.248

This change increases the sample size for the test to make it pass on the GPU.

PiperOrigin-RevId: 205436438