Better consumption of AAR during APK build.

GenerateSourcesMojo#generateR more closely follows AndroidBuilder way of building.

GenerateSourcesMojo#addResourceDirectories was modified to only add a single combined res folder as per AndroidBuilder. I'm unsure whether in doing so I have cut out something critical and whether this is correct/safe/good. But it's what AndroidBuilder does and it works.

ApklibMojo was changed so that it includes R.txt into APKLIB builds. This means we have a ready way of generating R for the APKLIB when creating the APK without having to invoke appt more than once.

 If we need to support APKLIB with no R.TXT then we can uncomment the #generateRForApklibDepedency method in ResourceClassGenerator and force it to work.

NB Have been testing this against the android-maven sample-libraryproject modified to include aar1-lib1 and librarypojects-lib2 as the 2 dependent libs of mainapp. I will generate a pull request for that too once I have added in a few more items that will better test all scenarios.