Add documentation link to wtf log

To allow developers easy access to documentation on background activity launches (goo.gle/android-bal), I've added a link to the relevant warning/error messages in the wtf log. This will make it easier for developers to understand the issue and find potential solutions.

Test: atest BackgroundActivityLaunchTest
Flag: EXEMPT logging only
Bug: 372918480
Change-Id: I3ee61afa683ebbb055ffa03dbe232f11cdd73ee6