Enable new initial-focus behavior for P

This means that, by default, nothing gets initial focus.

In order to have something focused at activity start, an
app must explicitly provide a <requestFocus /> tag. Since
this tag uses the "default" focus mechanism, it can be
placed in a parent and focus will go to whatever view is
the default focusable in that hierarchy.

This takes effect for targetApi >= P

Bug: 34520588
Test: Added CTS tests in View_FocusHandlingTest
Change-Id: I59fe2b9de2e0f7a21e3dd11e7a837b7c83f410f5