Enabled (asymmetric) tab selection in GridView

We decided its better for tab to do something than to have no
tab navigation within GridView.

Also fixed a small bug that made backwards focus order not work
right.

Bug: 38264959
Test: Manually tested in test-app. Added basic CTS for tab keys
Change-Id: I8236deed26e6d8b8cae0130359b104af4d9a244d