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.
Removed --debug-mode from aapt.
Fixed a bug that was causing the packaging to crash because it was looking for resources on both combinedRes folder as well as all other resource folders, so it was finding it twice.
Add compatibility for apklib & aar to be mixed together in an apk
Merge pull request #255 from Shusshu/aar-consumption-compat

Better consumption of AAR during APK build part 2.