Support gradle composite builds for local checkouts

For local ExoPlayer checkouts, the current approach is to include the relevant projects directly. For example:
`apply from: file("path/to/exoplayer/core_settings.gradle")`
and then
`implementation project(':exoplayer-library-core')`

The downside of this approach is that it makes it tricky to swap between the prebuilt binary dependency and the local checkout.

A better solution would be to support composite builds. See https://docs.gradle.org/current/userguide/composite_builds.html

With this approach, the dependencies are automatically substituted as necessary. The syntax is:
`includeBuild "path/to/exoplayer"`

Gradle automatically substitutes prebuilt library dependencies with the local checkout projects.