Do not show multiple footers if no usages are available.

We show a footer in PermissionAppsFragment if no permission usages are
available.  If you go to such a screen, click on an app, and go back,
a second footer appears.  This fixes that bug.

Bug: 129536503
Test: Go to page with no usages, see one footer, click on an app, go
back, still see one footer.

Change-Id: I8dd7741134f4fe007c6cd07b2549d6a94b4cbca4