Suggest building at Android API level 16.

Android >= L requires that binaries be position independent. However,
this is only enabled with API level 16 or above. Since developers are
likely to have modern Android versions, suggest API level 16 in the
BUILDING file to save someone in the future having to figure this out.

Change-Id: I66db7228e3d6fef0aa8dcfcfff67a71cb630a2b9

BUILDING

@@ -67,6 +67,7 @@ Then make a build directory as above and run CMake *twice* like this:
   cmake -DANDROID_NATIVE_API_LEVEL=android-9 \
         -DANDROID_ABI=armeabi-v7a \
         -DCMAKE_TOOLCHAIN_FILE=../util/android-cmake/android.toolchain.cmake \
+        -DANDROID_NATIVE_API_LEVEL=16 \
         -GNinja ..
 
 Once you've run that twice, ninja should produce Android-compatible binaries.