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
(cherry picked from commit a7fb0186)
Merged-In: I5eab8be5d87fdd9468bcbd8d54913ca713559314