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/ConsumerBase.h b/include/gui/ConsumerBase.h
index f7ab5ac..46ad20e 100644
--- a/include/gui/ConsumerBase.h
+++ b/include/gui/ConsumerBase.h
@@ -153,8 +153,8 @@
     // initialization that must take place the first time a buffer is assigned
     // to a slot.  If it is overridden the derived class's implementation must
     // call ConsumerBase::acquireBufferLocked.
-    virtual status_t acquireBufferLocked(IGraphicBufferConsumer::BufferItem *item,
-        nsecs_t presentWhen);
+    virtual status_t acquireBufferLocked(BufferItem *item, nsecs_t presentWhen);
+    virtual status_t acquireBufferLocked(BufferQueue::BufferItem *item, nsecs_t presentWhen);
 
     // releaseBufferLocked relinquishes control over a buffer, returning that
     // control to the BufferQueue.