Add a wifi-style progress bar to the Permissions Hub.

Use a linear progress bar (like on the wifi scan screen) when the
Permissions Hub is loading (except for the initial load, when we still
show the circular icon).  We hide the progress bar when it is not
visible so we keep the new app bar shadow behavior.

Bug: 123538370
Test: Open Permissions Hub, refresh it, click something, and go back.
Change-Id: I1a9dc7d831e387bfd917d16feacb11cdc448d256