libgui: Prepare for IGBC::BufferItem removal

Switches some dependencies from IGraphicBufferConsumer::BufferItem to
android::BufferItem and adds some methods to facilitate incrementally
changing client code to do the same.

Change-Id: I699ed0a6837076867ca756b28d1ffb2238f7a0d9
diff --git a/include/gui/GLConsumer.h b/include/gui/GLConsumer.h
index bf9eb24..167ef20 100644
--- a/include/gui/GLConsumer.h
+++ b/include/gui/GLConsumer.h
@@ -241,8 +241,8 @@
 
     // acquireBufferLocked overrides the ConsumerBase method to update the
     // mEglSlots array in addition to the ConsumerBase behavior.
-    virtual status_t acquireBufferLocked(BufferQueue::BufferItem *item,
-        nsecs_t presentWhen);
+    virtual status_t acquireBufferLocked(BufferItem *item, nsecs_t presentWhen);
+    virtual status_t acquireBufferLocked(IGraphicBufferConsumer::BufferItem *item, nsecs_t presentWhen);
 
     // releaseBufferLocked overrides the ConsumerBase method to update the
     // mEglSlots array in addition to the ConsumerBase.