Switch away from DistributionStrategy.fetch() (mostly just in tests)

so we can delete it. Frequently we can now delete the call entirely,
but in other cases we switch to read_var().

This revealed some bugs also fixed in this CL:
* For MirroredStrategy: fix read_var(mean_tower_local) bug.
* Support get() for Mirrored values that are not MirroredVariables,
  and make them DistributedDelegates so we can operate on them in
  cross-tower mode.
* Actually iterate through the available devices in MirroredStrategy.get().

With this and already-submitted 201390698, we can pass mirrored
variables and other mirrored values directly to self.evaluate() in
tests.

PiperOrigin-RevId: 201435436