Track libcore change 694e617f54a7bfbdad24913ce96f5d56f1a1960a

This change reverts previous fix for b/27582395 and makes
AbstractStringBuffer an Appendable. 

Bug: 28666126
Bug: 27582395 
Change-Id: I4b1b554fb47f45459eec24801af4ec798fc3039b
2 files changed