Explicitly set which taskbar view is visible at each caller

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