[issue-343] Allow copying the manifest during the generate source phase

Example configuration would be:
```xml
<sourceManifestFile>${project.basedir}/src/AndroidManifest.xml</sourceManifestFile>
<androidManifestFile>${project.build.directory}/AndroidManifest.xml</androidManifestFile>
```

Defaults (for now) to having sourceManifestFile not specified and thus using pre-existing behavior.
Copies sourceManifestFile to androidManifestFile during generate-sources phase.
Logic taken from ManifestUpdateMojo.