Separating project generation into @bubblewrap/core

The use case is using a Bubblewrap-generated project with a different build system (not Gradle).

Currently a big part of project generation resides in Gradle ([resources](https://github.com/GoogleChromeLabs/bubblewrap/blob/f48794d14d9c20892ed877574759f1b2b8b227ab/packages/core/template_project/app/build.gradle#L64-L150), [some manifest values](https://github.com/GoogleChromeLabs/bubblewrap/blob/f48794d14d9c20892ed877574759f1b2b8b227ab/packages/core/template_project/app/build.gradle#L59-L62), [shortcuts](https://github.com/GoogleChromeLabs/bubblewrap/blob/f48794d14d9c20892ed877574759f1b2b8b227ab/packages/core/template_project/app/build.gradle#L172-L205)), so you need an additional build step to finish creating a project usable with another build system, which makes it necessary to have JDK and Android SDK available at project generation time.

The resource files and Android manifest could be templated with lodash directly, bypassing Gradle. Is this something you'd be open to?