Fix taskbar stash state not persisting properly across recreate

Because we check supportsVisualStashing() in
TaskbarStashController#init(), we need to avoid using
TaskbarUIController to provide that value since TaskbarUIController
isn't initialized until a bit later than the other controllers. So I
moved the logic from supportsVisualStashing() back to
TaskbarStashController, but still allow TaskbarUIController to override
it (e.g. for DesktopTaskbarUIController).

After that fix, I noticed that force stopping launcher (to test the fix)
would briefly show the taskbar background before resetting the stashed
state. This is also due to LauncherTaskbarUIController not being ready
immediately, since that's what sets FLAG_IN_APP due to launcher being
paused. To work around this, I set FLAG_IN_APP to true by default in
TaskbarStashController#init(), since that is the most common case, and
taskbar background/stashed handle isn't shown on home anyway.

Test: Force stop launcher while taskbar is stashed, verify it recreates
as stashed without background flicker; same when changing wallpaper
color on home or in app; also tested when taskbar isn't stashed and in 3
button mode for good measure
Test: testHideTaskbarPersistsOnRecreate
Fixes: 235986838

Change-Id: Ie55bd70e8288d5ad7433dde970f18c176831d747