Fix google-build-using-namespace warnings in cpu_ref.

* Remove "using namespace ..." statements.
* Replace them with using declarations of the required names.
* Enclose the C++ methods and static and extern "C" functions with
  namespace android and renderscript.
* Keep global C++ functions as-is and add using declarations for them.

Bug: 32670901
Test: build with WITH_TIDY=1
Change-Id: I818de466e8786a6c4f9ce0cd8e0fe027f34d7fad
diff --git a/cpu_ref/rsCpuRuntimeMath.cpp b/cpu_ref/rsCpuRuntimeMath.cpp
index b416462..6a8f7b6 100644
--- a/cpu_ref/rsCpuRuntimeMath.cpp
+++ b/cpu_ref/rsCpuRuntimeMath.cpp
@@ -27,8 +27,9 @@
 #include "rsCpuCore.h"
 #include "rsCpuScript.h"
 
-using namespace android;
-using namespace android::renderscript;
+using android::renderscript::Matrix2x2;
+using android::renderscript::Matrix3x3;
+using android::renderscript::Matrix4x4;
 
 #define EXPORT_F32_FN_F32(func)                                 \
     float __attribute__((overloadable)) SC_##func(float v) {    \