Add content_shell_test_apk and a several dependencies

content/public/test/android/BUILD.gn

+import("//build/config/android/config.gni")
+import("//build/config/android/rules.gni")
+
+# GYP: //content/content_tests.gypi:test_support_content_jni_headers
+generate_jni("test_support_content_jni_headers") {
+  jni_package = "content/public/test"
+  sources = [
+    "javatests/src/org/chromium/content/browser/test/NestedSystemMessageHandler.java",
+  ]
+}
+
+# GYP: //content/content_tests.gypi:content_java_test_support
+android_library("content_java_test_support") {
+  deps = [
+    "//base:base_java",
+    "//base:base_java_test_support",
+    "//content/public/android:content_java",
+  ]
+  DEPRECATED_java_in_dir = "javatests"
+}