AIOOB exception fix in TabWidget

Bug http://code.google.com/p/android/issues/detail?id=15005

The problem was not specific to the legacy theme. The code that first
measure the tab's width with no contraint was incorrectly using the
mImposedTabsWidth array which could not have the right size if a
child was added.

The first measure after a child is added should indeed crash. Could
be investigated. This fix is sure anyway.

Change-Id: I4c0594e92019aabc545619e874bdfd49ce69c90e