[sysui] Use weak reference for the listener in NavigationBarTransitions

The listener is the only thing keeping objects alive after
switching dark theme, bringing the number of Resources.Theme
objects to thousands.

Bug: 242005877
Test: manual, 10k theme switches to verify there's fewer active
      theme objects
Change-Id: I59ff27dacd4bcad933341886c249074cf7df6dc9
(cherry picked from commit 0776b3b4)