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

As the current tab is not set from application, getChildTabViewAt
returns Null value. Added a condition which checks whether mSelectTab
is set to first Tab.

(cherry picked from commit 9be6755efb149b22c36aa3eb7cc4ffe7e765e08a)

Change-Id: I11c77ca96333e2663d28db1b5fdb46602bd9e745
CRs-Fixed: 321176