[Thread] move createRandomDataset to ThreadNetworkController

This commit renames `createRandomDataset` to `createRandomizedDataset`
and moves this method to ThreadNetworkController class.

The reason for moving this method is that we rely on the service side to
know which channels are currently supported by this device and there is
no safe way for ActiveOperationalDataset to created a randomized full
dataset by its own. Moving this method to ThreadNetworkController also
allows us to support future platform-dependend configurations.

Bug: 308117328
Bug: 307939803
Change-Id: I0b0b7b727c3b326ab59bb1299ace15df1ff1ad5e