Fix android.text.cts.TextUtilsTest#testRegionMatches

The CTS test expects an ArrayIndexOutOfBounds exception when passing in
an unreasonably large value for len. Since the actual implementation was
causing an integer overflow, we were getting a different exception.
Since integer overflow is potentially dangerous, this patch tests for it
and throws an exception explicitly.

Change-Id: I0420c06185d33d130853861d25d4f65b06fe0dfa