Add build.gradle to easily modifying Java project with Android Studio

Change-Id: Ie030ff0a3569caa0eacfa331d21f59e5455c90ed
Reviewed-by: Andy Shaw <andy.shaw@qt.io>
(cherry picked from commit 886a2d4511bdd5705595bcf4a29fb22f84c0e9aa)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/android/jar/.gitignore

@@ -0,0 +1,6 @@
+.gradle/
+build/
+gradle/
+gradlew
+gradlew.bat
+local.properties

src/android/jar/AndroidManifest.xml

@@ -1,5 +1,4 @@
 <?xml version='1.0' encoding='utf-8'?>
 <manifest xmlns:android="http://schemas.android.com/apk/res/android" android:versionCode="1" android:versionName="1.0" package="org.qtproject.qt.android">
     <supports-screens android:largeScreens="true" android:normalScreens="true" android:anyDensity="true" android:smallScreens="true"/>
-    <uses-sdk android:minSdkVersion="9" />
 </manifest>

src/android/jar/build.gradle

@@ -0,0 +1,54 @@
+// This is mainly used to allow Android Studio to easily read this folder as an android project.
+
+buildscript {
+    repositories {
+        google()
+        jcenter()
+    }
+
+    dependencies {
+        classpath 'com.android.tools.build:gradle:4.0.2'
+    }
+}
+
+apply plugin: 'com.android.library'
+
+dependencies {
+    implementation fileTree(dir: "libs", include: ["*.jar"])
+//     implementation 'androidx.appcompat:appcompat:1.2.0' // for androidx
+}
+
+repositories {
+    google()
+    jcenter()
+}
+
+android {
+    compileSdkVersion 30
+    buildToolsVersion "30.0.3"
+
+    defaultConfig {
+        minSdkVersion 23
+        targetSdkVersion 30
+    }
+
+    sourceSets {
+        main {
+            java.srcDir 'src/'
+            resources.srcDir 'libs/'
+            manifest.srcFile 'AndroidManifest.xml'
+            res.srcDirs = ['res/']
+        }
+    }
+
+    compileOptions {
+        sourceCompatibility JavaVersion.VERSION_1_8
+        targetCompatibility JavaVersion.VERSION_1_8
+    }
+
+    android {
+        lintOptions {
+            abortOnError true
+        }
+    }
+}

src/android/jar/settings.gradle

@@ -0,0 +1 @@
+rootProject.name = "QtAndroid"