Partially migrate KSP off AGP's legacy Variant API

[scott-pollom]

Use AGP's new addKspConfigurations() API when possible.

Bug: #2250
Test: AGP741IT and existing ProcessorClasspathConfigurationsTest

[ting-yuan]

This seems to strongly depend on the implementation of AndroidComponentsExtension.addKspConfigurations. Are there any guarantees or documentation of what configurations it creates / uses, or slightly more: how does KSP's Gradle plugin and AGP work together? For example, it creates / uses KSP configuration like kspDebug / kspRelease / etc, and corresponding kspDebugProcessorClasspath / etc. Without a formal contract or at least tests in AGP it is quite vulnerable to future changes in both side.

[scott-pollom]

There is a test on the AGP side here: https://cs.android.com/android-studio/platform/tools/base/+/mirror-goog-studio-main:build-system/integration-test/application/src/test/java/com/android/build/gradle/integration/application/ConfigurationPerSourceSetTest.kt;l=162

And there is a test on the KSP side in ProcessorClasspathConfigurationsTest.kt

AGP's API creates the same configurations previously created by KSP. The only difference is that it creates the ...KotlinProcessorClasspath configurations eagerly.