frameworks/base: Fix to avoid crash when the tab is not set.

Tab is not highlighted in Videoapp unlike Music app.
Instead the list is highlighted with mSelectedTab a negative value
when onFocusChange is called.
Add a check for mSelectedTab.

Change-Id: I930657b8a9be4be4d973f876242a15a667ec6f94
CRs-Fixed: 657606