Keeping sp<IBinder> around in ClientCache

binderDied is no longer called if all sp references are dropped in SF
process.  Keep an sp<IBinder> around so that we can get notified to
clean up the ClientCache/RenderEngine

Bug: b/144371238
Test: build, boot, manual (script to open and close Chrome
repeatedly, check RenderEngine image cache size in dumpsys), SurfaceFlinger_test,
libsurfaceflinger_unittest

Change-Id: Ib9374c02db59e09ca53280acc64419bda6827a4d