Simplify the protocols for 'constant' bindings

[copybara-service]

Simplify the protocols for 'constant' bindings

• accept an InternalContext parameter from Initializable.get which all callers can provide.
  • This speeds up the injectAll phrase of injector creation since we no longer need to create an InternalContext object for each instance binding.
• for 'constant' bindings with no injection points use a simpler set of InternalFactory types.
  • This provides some immediate benefits by eliminating indirection (though admittedly the impacts are minor), but when we adopt method handles this will enable a cheaper implementation there as well.