This CL makes TaskContentView focusable and adds a focus border to it
that is shown when it has focus. There are some caveats currently:
* Recents has a bug where it starts to eat TAB inputs (b/403292849).
This means that the tab navigation is currently quite shaky and
often just works the first time entering recents.
* Activating a window (with enter) is currently disabled because of
another existing bug (b/413378320).
Flag: com.android.launcher3.enable_desktop_exploded_view
Test: Manual
Bug: 353949138
Change-Id: I1c9c7e437f2e131c4ead1d750952ee9ff77c17f4
04066346c2