make androidx.startup Initializers public

[fbarthelery]

This allows to use LeakCanary initializers as dependencies using the dependencies() method. As this is an advanced use case and it should not be needed for basic usage, I kept the classes in leakcanary.internal package.

Fixes #2393

[CLAassistant]

All committers have signed the CLA.

[pyricau]

As this is an advanced use case and it should not be needed for basic usage, I kept the classes in leakcanary.internal package.

I'd still like to stay consistent and have only internal APIs inside the internal package. Can you move the classes up one level? You'll likely need to update manifest references as well. Thanks!