The API `createRandomizedDataset()` randomly selects a channel
between channel 11 and 26. But not all countries support channels
between 11 and 26.

This CL selects random channel from  the preferred channel mask if
the preferred channel mask is not 0, otherwise choose one from the
supported channel mask.

Bug:b/308310823
Test: atest ThreadNetworkUnitTests && atest CtsThreadNetworkTestCases

Change-Id: I6add3edc7e4fa888c46b57c4463ab18e0aa84e80