Make new ProcessState member const.

Bug: 129785390
Test: builds
Change-Id: Iea4de80fdeb8395216094f9230d2b0f9a9c92c24
(cherry picked from commit 418856e083a27ef19744b12208c49c84b0bec1d0)
diff --git a/libs/binder/include/binder/ProcessState.h b/libs/binder/include/binder/ProcessState.h
index 8dee3ad..8a1f7e2 100644
--- a/libs/binder/include/binder/ProcessState.h
+++ b/libs/binder/include/binder/ProcessState.h
@@ -138,7 +138,7 @@
             String8             mRootDir;
             bool                mThreadPoolStarted;
     volatile int32_t            mThreadPoolSeq;
-            size_t              mMmapSize;
+            const size_t        mMmapSize;
 
             CallRestriction     mCallRestriction;
 };