Update test for non-nested PINNED tasks

Was expecting old behavior of nested task in pinned, but that
isn't how it works anymore.

Added a test to make sure that pinned tasks are replaced when
using ActivityOptions to enter PINNED mode.

Bug: 153201813
Test: PinnedStackTests
Change-Id: I5ae05cafe655822b70af9e3ab0fd24ed31e10316