Refactor "app plugin loader" Gradle Plugin so it can be applied using the declarative plugins {} block

[bartekpacia]

This PR fixes #125009.

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[bartekpacia]

module_plugin_loader should be refactored like this as well.

[bartekpacia]

Problem 1: I cannot get com.android.tools.build:gradle into FGP's runtime classpath.

Problem 2: implementation('com.android.tools.build:gradle:xxx') isn't resolved for some strange reason (aside from that we shouldn't probably use implementation at all here)

[gmackall]

Dismissing review from @dnfield as stale (all points were addressed), so I can get autosubmit to stick.

It has been addressed