Removes existing singleInstance Activity when starts as home

... or there would have two instance of the singleInstance
activity. We don’t do it the other way around to prevent abuse.

Looks like this was accidentally changed via commit 28d8042b.

Bug: 334005805
Test: verified the behavior via MixedDeviceOwnerTest
Test: verified the power hint not send when restarting home
Change-Id: I463da7efa3f3d367d62ac59cbac98aa5804f8615