Bring back the status bar activity indicators for mobile data and wifi

Google were nice enough to create xxhdpi drawables for us before removing them.

Reverts "Remove signal/wifi activity indicator icon overlays."

This reverts commit fcc901c10bffdfb9c6b31e676dec36b47d949979.

Change-Id: I7aa85f7c8442c45d17e42cac7823dfe93e3f24c1
(cherry picked from commit 69f39661d63f920e6d1b65428b175b064925b910)