Fix leak of WeakReferences on mThemeRefs list.

Periodically remove references from the list whose referents have been
garbage collected.

Bug: 73961798
Test: Device boots.
Test: Take a heap dump of systemui and manually check that the state of
      ThemeRefs looks reasonable.

Change-Id: I691027feb5dd217bcb60406b28897b9614e2a845
1 file changed