Provide old behavior for View#hasFocusable to old apps

Existing apps have copies of ListView behavior that expects the old
hasFocusable behavior of only reporting explicitly focusable views.
Use targetSdkVersion to give the old behavior to existing apps.
Add hasExplicitFocusable method to View to offer the old behavior
to apps that need it. ViewCompat method required in support library
to shim through to hasFocusable pre-O.

Bug: 34946284
Test: CTS
Change-Id: I65503fa2c5fe4cb8b635efa464e653bbf5e7f821