Remove Gradle as build system

This application must be compiled with the operating system, so Gradle is unnecessary.

Android.mk

@@ -21,6 +21,6 @@ LOCAL_PACKAGE_NAME := Backup
 LOCAL_MODULE_TAGS := optional
 LOCAL_REQUIRED_MODULES := com.stevesoltys.backup.xml
 LOCAL_PRIVILEGED_MODULE := true
-LOCAL_SRC_FILES := $(call all-java-files-under, java)
+LOCAL_SRC_FILES := $(call all-java-files-under, src)
 LOCAL_RESOURCE_DIR := $(LOCAL_PATH)/res
 include $(BUILD_PACKAGE)

app/build.gradle

@@ -1,17 +0,0 @@
-apply plugin: 'com.android.application'
-
-android {
-    compileSdkVersion 26
-    buildToolsVersion '26.0.1'
-
-    buildTypes {
-        release {
-            minifyEnabled false
-            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
-        }
-    }
-}
-
-dependencies {
-    compile fileTree(include: ['*.jar'], dir: 'libs')
-}

build.gradle

@@ -1,23 +0,0 @@
-// Top-level build file where you can add configuration options common to all sub-projects/modules.
-
-buildscript {
-    repositories {
-        jcenter()
-    }
-    dependencies {
-        classpath 'com.android.tools.build:gradle:2.3.3'
-
-        // NOTE: Do not place your application dependencies here; they belong
-        // in the individual module build.gradle files
-    }
-}
-
-allprojects {
-    repositories {
-        jcenter()
-    }
-}
-
-task clean(type: Delete) {
-    delete rootProject.buildDir
-}

settings.gradle

@@ -1 +0,0 @@
-include ':app'