Pausing the resumed activity when making it invisible

A resumed activity was added to stopping activity lists while
making it invisible without having state changes. So, the
activity won't be resumed if it became visible again because
the activity is already in resumed state.

Also adding the reason of why the activity should be paused.

Bug: 160435184
Test: atest ActivityRecordTests
Change-Id: Ie92fdef045cb0ba06fee9575878f8849f7eda1d9