Fix Java tests for older Gradle versions

Before this, Gradle 4.4 was required to build the Gradle wrapper and
thus run the Java tests. This was because of all the stuff that had to
be configured when running the wrapper task using the build.gradle file.

Now we use a config file that only specifies the required Gradle version
and nothing else to run the wrapper task. This allows tests to be run
with Gradle versions beginning with 2.

test/run_java_unit_tests.js

@@ -26,7 +26,7 @@ var superspawn = require('cordova-common').superspawn;
 // First we make sure the gradlew helper file is built and ready.
 var GradleBuilder = require('../bin/templates/cordova/lib/builders/GradleBuilder');
 var builder = new GradleBuilder(__dirname);
-var needs_gradlew_built = builder.runGradleWrapper('gradle', 'build.gradle');
+var needs_gradlew_built = builder.runGradleWrapper('gradle');
 
 if (!needs_gradlew_built) {
     // Due to interface of gradle builder, if the gradlew file already exists, `runGradleWrapper` returns undefined.

test/wrapper.gradle

@@ -0,0 +1,3 @@
+wrapper {
+    gradleVersion = '4.4.1'
+}