Don't keep a list of AutoHideUiElements

Running into memory leaks in sysui due to this approach. Revert to
original approach (member variables for necessary system bars) for now
and revisit maintaining a list when necessary.

Bug: 150961453
Test: `atest NexusLauncherTests` and `adb shell am dumpheap <sysui pid> <file>` before and after running test
Change-Id: I7b42fa5acbd0f66f3249e5cad8d1747642c4ab21