Enable kotlin's jvm-abi-gen plugin to generate header jars

Kotlin's jvm-abi-gen plugin can generate header jars similar to the
turbine output for java sources, which can be used to avoid recompiling
downstream modules when the ABI hasn't changed.  Unlike turbine, the
plugin runs as part of the main kotlinc invocation, so it doesn't allow
the downstream modules to start compiling any sooner.

A future possible optimization is to use turbine to compile the kapt
stubs, at least for the  kotlin+annotation processor modules that already
generate them, which would allow compiling downstream modules before
invoking kotlinc or javac, as well as invoking kotlinc and javac in
parallel with each other.

Bug: 222095735
Test: TestKotlin
Test: m SystemUI
Change-Id: Ib1bb2ecea47c851a108a26f9ed4f827f289d1321

java/config/kotlin.go

@@ -34,6 +34,7 @@ func init() {
 	pctx.SourcePathVariable("KotlinKaptJar", "external/kotlinc/lib/kotlin-annotation-processing.jar")
 	pctx.SourcePathVariable("KotlinAnnotationJar", "external/kotlinc/lib/annotations-13.0.jar")
 	pctx.SourcePathVariable("KotlinStdlibJar", KotlinStdlibJar)
+	pctx.SourcePathVariable("KotlinAbiGenPluginJar", "external/kotlinc/lib/jvm-abi-gen.jar")
 
 	// These flags silence "Illegal reflective access" warnings when running kapt in OpenJDK9+
 	pctx.StaticVariable("KaptSuppressJDK9Warnings", strings.Join([]string{