Update docs for onBackPressed

After changing the default behavior of the back button in S, we should
update the javadocs to reflect the change.

We also make a recommendation for developers to call through to the
super implementation to maintain consistency in navigation patterns
throughout the platform, especially for things like splash screens.

Bug: 186142540
Bug: 188832670
Test: None
Change-Id: I1c71250b26bec743be15f4ac95965ef046dcf4f3