Add edgeEffectType attribute and type property

Bug: 178807038

This adds the edgeEffectType property and
type attribute. These will be used to
enable stretchy edge effect for overscroll.

Test: new CTS test for setting/retrieving the effect type.
Test: Ic363f71d241649d15faa86434516457e62b261f4
Change-Id: I16f20b36d9a15c08e0a37f98572ef9c9f8fbc88f