DO NOT MERGE Use round highlight for navigation buttons

Make the focus highlight and touch ripple for buttons in the navigation
bar be round rather than square.

Also fix a bug in the dimming logic. The alpha was applied to the icon
initially, but when the state changed, the alpha was applied to the
entire button.

Launcher in master is quite different so it this change is limited to
rvc-qpr-dev for now.

Test: manual
Bug: 174614908
Bug: 168317872
Change-Id: I9b80946887ad85a73ddd281f7afcc9fcc8280a64