libbinder: avoid attached objects for stability

Attached objects require a heap allocation, and stability is always
attached. This avoids a heap allocation per binder object (Android R
regression).

Bug: 148177595
Test: binderStabilityTest

Change-Id: I5eab8be5d87fdd9468bcbd8d54913ca713559314
diff --git a/libs/binder/Binder.cpp b/libs/binder/Binder.cpp
index 2f6e9c3..e0fb543 100644
--- a/libs/binder/Binder.cpp
+++ b/libs/binder/Binder.cpp
@@ -141,7 +141,7 @@
 
 // ---------------------------------------------------------------------------
 
-BBinder::BBinder() : mExtras(nullptr)
+BBinder::BBinder() : mExtras(nullptr), mStability(0)
 {
 }