chore: Replace JavaExec.main property with JavaExec.mainClass

The main property is deprecated and should be replaced with mainClass
which takes a Property<String> instance (created behind the scenes by
using '=') instead of a string.

test-app/app/build.gradle

@@ -501,7 +501,7 @@ task runSbg(type: BuildToolTask) {
     inputs.dir(extractedDependenciesDir)
 
     workingDir "$BUILD_TOOLS_PATH"
-    main "-jar"
+    mainClass = "-jar"
 
     def paramz = new ArrayList<String>()
     paramz.add("static-binding-generator.jar")
@@ -769,7 +769,7 @@ task buildMetadata(type: BuildToolTask) {
     outputs.files("$METADATA_OUT_PATH/treeNodeStream.dat", "$METADATA_OUT_PATH/treeStringsStream.dat", "$METADATA_OUT_PATH/treeValueStream.dat")
 
     workingDir "$BUILD_TOOLS_PATH"
-    main "-jar"
+    mainClass = "-jar"
 
     doFirst {
         // get compiled classes to pass to metadata generator
@@ -864,7 +864,7 @@ task generateTypescriptDefinitions(type: BuildToolTask) {
     def includeDirs = ["com.android.support", "/platforms/" + android.compileSdkVersion]
 
     workingDir "$BUILD_TOOLS_PATH"
-    main "-jar"
+    mainClass = "-jar"
 
     doFirst {
         delete "$TYPINGS_PATH"