Simplify InlineSwitchPayloads and generalize get/set method

InlineSwitchPayload now assumes that all switches will be
stored as 1 or 0, which simplifies the code.

Moves Availability into ResultPayload, so that custom
payloads can be created to set/get values which are more
complicated than stotring ints (like bluetooth or DnD),
and give more expressive reasons when unavailable.

Bug: 62022517
Test: make RunSettingsRoboTests
Change-Id: I87e6fc041bfd398e7daf6e6e479d502930d36f0f