libgui: Normalize IGraphicBufferConsumer methods

This change modifies IGraphicBufferConsumer such that all synchronous
methods return a status_t and, as such, all outputs are returned by
output parameter rather than return value. It also eliminates the use
of a const char* parameter that SafeInterface can't handle.

Test: m -j + libgui_tests + manual testing
Change-Id: I9bb0e2892492d5e0a3f660bf228834b9ed014539
diff --git a/libs/gui/ConsumerBase.cpp b/libs/gui/ConsumerBase.cpp
index d4e4dc3..1783561 100644
--- a/libs/gui/ConsumerBase.cpp
+++ b/libs/gui/ConsumerBase.cpp
@@ -270,7 +270,9 @@
     result.appendFormat("%smAbandoned=%d\n", prefix, int(mAbandoned));
 
     if (!mAbandoned) {
-        mConsumer->dumpState(result, prefix);
+        String8 consumerState;
+        mConsumer->dumpState(String8(prefix), &consumerState);
+        result.append(consumerState);
     }
 }