Added support for android.R.attr#alwaysFocusable

Allows an activity to always be focusable regardless of if it is in a
stack whose activities are normally not focusable. For example, activities
in pinned stack aren't focusable. This flag allows them to be focusable.

Also, changed ActivityInfo.#{resizeable, supportsPip} to use flags.

Bug: 26273032
Bug: 26034613
Change-Id: I8c63e6d3256757e2e6931e08b8a65269f5169d35