Only inset IME by 48dp instead of 60dp when taskbar is present

- Update providedInternalImeInsets whenever taskbar height changes, since the insets are subtracted from the taskbar window height. This ensures the IME always has the same bottom inset (48dp) regardless of taskbar's window size
- Also translate nav buttons down to center in the new size when IME is showing

Test: Open IME from home and in apps, both in gesture nav and 3 button mode. Ensure IME bottom inset is always 48dp and nav buttons center within that space
Fixes: 201115344
Change-Id: I5ed25b0ffc08145b2221cc5c960e669cc21b2aa7