[TF:XLA] Refactor XlaDevice construction.

a) remove the XlaDevice::Create() factory function, instead have users call the
   constructor directly. Using a factory function makes it difficult to subclass
   XlaDevice, which is useful to carry along extra backend-specific information as part
   of the device.
b) move the arguments to the constructor into an XlaDevice::Options structure to improve
   readability and allow backends to only set the relevent subset of the options.

No functional changes intended; out-of-tree TF/XLA backends will need to perform a small refactoring (see the changes to xla_cpu_device.cc for an example.)

PiperOrigin-RevId: 217618620