Change some globals to static

Before:
   text	   data	    bss	    dec	    hex	filename
1875636	 128944	   9624	2014204	 1ebbfc	libandroid_runtime.so

After:
   text	   data	    bss	    dec	    hex	filename
1875403	 128920	   9624	2013947	 1ebafb	libandroid_runtime.so

Test: build
Change-Id: Ie1d00b288c8411c8af012f3b9d7e3b740ddd3247
diff --git a/core/jni/android_util_AssetManager.cpp b/core/jni/android_util_AssetManager.cpp
index af47cb6..c7b36d0 100644
--- a/core/jni/android_util_AssetManager.cpp
+++ b/core/jni/android_util_AssetManager.cpp
@@ -108,7 +108,7 @@
   jmethodID put;
 } gArrayMapOffsets;
 
-jclass g_stringClass = nullptr;
+static jclass g_stringClass = nullptr;
 
 // ----------------------------------------------------------------------------