Fix regression where nav bar shows when taskbar is showing

- Add mNavBarButtonAlpha to TaskbarStateHandler, and set it to the inverse of the taskbar alpha.
- Add mPendingSetNavButtonAlpha for when setNavBarButtonAlpha() is called before mSystemUiProxy is initialized.

Test: Go between various launcher states and ensure nav bar only shows up when taskbar hides.
Bug: 187353581
Change-Id: I4b79280d686c787e9128e753070ec591d1ab2e4d