Allow use of bindProcessToNetwork only to change private DNS bypass.

Currently it is not possible to change private DNS bypass by
doing:

  setProcessDefaultNetwork(network.getPrivateDnsBypassingCopy());
  setProcessDefaultNetwork(network);

because the code will ignore the change. Fix this by ensuring
that we always call bindProcessToNetwork (which does not have
side effects) and then only performing the expensive operations
(flushing DNS cache, upating socket pools) if the netId changed.

Bug: 112869080
Test: None
Change-Id: I5e8999cb11d8b8c1e9eb583fa8b3932f212accff