Keep stack from mReuseTask

When we launch an activity with CLEAR_TOP and NEW_TASK, we clear
all the activities in that task but then we don't reuse the stack,
which can lead to weirdness.

To fix this, we need to keep the target stack in that case.

Bug: 28000613
Change-Id: If85ade73b1036e8a7c0eda418d9f39e8596b8b16