Split the Android platform java logic into an Android library module (lib) and an application module (app).

The application module `app` serves double duties of providing the prebuilt Godot binaries ('android_debug.apk', 'android_release.apk') and the Godot custom build template ('android_source.zip').

platform/android/java/lib/CMakeLists.txt

@@ -0,0 +1,18 @@
+cmake_minimum_required(VERSION 3.6)
+project(godot)
+
+set(CMAKE_CXX_STANDARD 14)
+set(CMAKE_CXX_STANDARD_REQUIRED ON)
+set(CMAKE_CXX_EXTENSIONS OFF)
+
+set(GODOT_ROOT_DIR ../../../..)
+
+# Get sources
+file(GLOB_RECURSE SOURCES ${GODOT_ROOT_DIR}/*.c**)
+file(GLOB_RECURSE HEADERS ${GODOT_ROOT_DIR}/*.h**)
+
+add_executable(${PROJECT_NAME} ${SOURCES} ${HEADERS})
+target_include_directories(${PROJECT_NAME}
+        SYSTEM PUBLIC
+        ${GODOT_ROOT_DIR}
+        ${GODOT_ROOT_DIR}/modules/gdnative/include)