Remove confusing WindowState#getRootTask

It is weird to return home task for non-activity window. Also a
window usually should not care about whether its task is "root".

The legacy dim case of system window was handled by the dimmer on
display area.

Bug: 163976519
Flag: EXEMPT clean up legacy logic
Test: atest WindowStateTests#testCanBeImeTarget \
            testNeedsRelativeLayeringToIme_systemDialog
Change-Id: I3639d60cc180ca823fbb0c532e132a998f87a767