Pass button nav long press assistant availability to Launcher

* Currently we're passing in assistantAvailability, but that's
specific to gesture nav. For button nav even if assistant
isn't available, we show the UI to encourage user to sign in to
enable assistant
* Separate signals used for button vs gesture nav

Bug: 255909545
Test: Existing tests pass, updated/added test in launcher
Change-Id: Id8fba0d07826ff1013a078f81512d44d3d88a1ab