Make TestActivity.isInMultiWindowMode() safe

Override isInMultiWindowMode() in TestActivity to wrap the "super." call
in a try-catch block, since may cause NPEs on some platforms for some
OEMs when running DocumentsUIUnitTests.

Bug: 297710004
Test: atest DocumentsUIUnitTests
Change-Id: I5160cf54678eb8046f090b721557ae9d39527c65