This makes it more clear which taskbar view is expected for particular
code flows. Also makes it easier to set it to null in cleanup().
Test: call cleanup() while taskbarOnHome is invisible, ensure hotseat
icons go back to being visible
Bug: 182512211
Fixes: 183633496
Change-Id: I680a15582c8034a049b20e035b4d54f5ea5aee04