libgui: Change detachNextBuffer to return sp<GB>

Changes Surface::detachNextBuffer to return an sp<GraphicBuffer>
instead of an ANativeWindowBuffer* to ensure that reference counting
works correctly.

Bug: 20092217
Change-Id: I3979ea6121aaf14845f0554477b778770413581e
(cherry picked from commit 8b2daa3ca29492b181fb67840640d771c4a2b3ac)
2 files changed