--non-constant-id for generating R for APKLIBs

Adds the `--non-constant-id` flag to aapt when generating the R.class for
APKLIBs to give non-final ints. This is additional to the fix by xian
which was only doing this in `generateRForApkLibDependency` method - this
change does it in the `generateR` method as well but only for APKLIBs.

This is necessary for when you want to make an apklib that has your source
as a .jar but also contain some XML resource files. It doesn't fix [Issue
313](https://code.google.com/p/maven-android-plugin/issues/detail?id=313) but it basically allows you to do what the person who raised it
wanted.

**Note:** I tried doing a `mvn clean install` on the sample project (first without my change) and I could everything to except the `mixed-java-native-with-apklib-dependency` module in the `native` samples. By skipping this module it built fine and also then built fine with my change. I do confess to being quite new to both Maven and Git/GitHub so I'll apologise now if I've done anything obviously stupid!