Restore (limited) dynamic feature capability + introduce sample app

[oliviernotteghem]

This PR fixes (presumably obsolete) application bundle code :

• add missing declaration of xmllint tool target. This tool is used during dynamic feature manifest.xml validation.
• expose android_feature_module rule, so that it can be used in application bundle project.
• add missing fused attribute. This attribute is used internally by android_feature_module rules and has to be declared.
• fixes paths computation done by android_feature_module. Using Label() doesn't account for external usage.
• fixes _merge_manifests default attribute value used by android_application. Current value is bogus.
• change permissions of shell scripts used for validation purposes. They need to be executable for Bazel to run them.

A sample application is also provided to build a simple application bundle, whose dynamic feature contain one asset file (assets.txt). To build it : cd examples/bundle && bazel build app:assets

[ahumesky]

Thanks, I've started importing this so we can have it for the 0.6.0 release

[ahumesky]

One issue is that the bundle manifest validator relies on xmllint to extract info from the bundle manifest, and that tool is not installed on bazelci runner images (and we probably shouldn't depend on locally installed tools anyway). So we either have to find a way to get xmllint (for linux, mac, and windows), or build the minimal functionality for extracting the xml info into something in rules_android (ak probably)