Add the 'hello-neon' sample app to demonstrate cpufeatures and ARM Neon usage.

+ Add a small cleanup script (build/tools/cleanup-apps.sh)
+ Minor formatting of hello-gl2 sample.

ndk/apps/hello-gl2/project/jni/gl_code.cpp

@@ -42,12 +42,14 @@ static void checkGlError(const char* op) {
     }
 }
 
-static const char gVertexShader[] = "attribute vec4 vPosition;\n"
+static const char gVertexShader[] = 
+    "attribute vec4 vPosition;\n"
     "void main() {\n"
     "  gl_Position = vPosition;\n"
     "}\n";
 
-static const char gFragmentShader[] = "precision mediump float;\n"
+static const char gFragmentShader[] = 
+    "precision mediump float;\n"
     "void main() {\n"
     "  gl_FragColor = vec4(0.0, 1.0, 0.0, 1.0);\n"
     "}\n";