Allow using config to disable creation of navBar and taskBar

Some platforms, like ARC, has their own implementation of navBar and
taskBar, and do not require the creation of these instances.
Previously, config_showNavigationBar was implemented to control the
creation of navBar. Now, with this CL, we are also using the same config
to guard the creation of taskBar. This provides a platform-wide
configuration option to disable the creation of both navBar and taskBar
without introducing any divergence from the upstream code.

Bug: 271960221
Test: Build

Change-Id: Ieb86c0f1ed99075ba2417f9ba1c3fa7a603718ae