Support gradle composite builds for local checkouts

[sailesha]

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.

[icbaker]

This does sound good - would you be interested in sending a PR switching to it? (it would actually be great to send it to the main branch of the androidx/media GitHub project instead of this one, it will be easier for us to merge (and it will get pushed back out to here, the two repos are kept roughly in sync with each other)).