Update Android java code (#7820)

Targets SDK 26 as required by the playstore.
Fixes screen auto-rotation closing game.
Hides on-screen navigation bar if present.

Update gradlew.
Fix display aspect on 18+/:9 displays (like a Samsung Galaxy S9).
Remove small app icons, not required.
Fix xml in unpacking activity.
Support Android permission: On Android 6.0+ you need to manually give write
permission (as required by google).
Background during unpacking (just a demo for now).
Material Design: no more Android 2 interface.
Immersive mode (Android 4.4+ - hide NavBar for fullscreen mode).

build/android/build.gradle

@@ -1,25 +1,39 @@
 buildscript {
 	repositories {
-		mavenCentral()
+		maven { url 'https://maven.google.com' }
+		jcenter()
 	}
 	dependencies {
-		classpath "com.android.tools.build:gradle:2.3.0"
+		classpath 'com.android.tools.build:gradle:3.1.3'
+	}
+}
+
+allprojects {
+	repositories {
+		maven { url 'https://maven.google.com' }
+		jcenter()
 	}
 }
 
 apply plugin: "com.android.application"
 
 android {
-	compileSdkVersion 26
-	buildToolsVersion "26.0.3"
+	compileSdkVersion 28
+	buildToolsVersion "28.0.3"
 
 	defaultConfig {
 		versionCode 21
 		versionName "${System.env.VERSION_STR}.${versionCode}"
 		minSdkVersion 14
-		targetSdkVersion 14
+		targetSdkVersion 28
 		applicationId "net.minetest.minetest"
-		manifestPlaceholders = [ package: "net.minetest.minetest", project: project.name ]
+		manifestPlaceholders = [package: "net.minetest.minetest", project: project.name]
+		ndk {
+			// Specifies the ABI configurations of your native
+			// libraries Gradle should build and package with your APK.
+			abiFilters 'armeabi-v7a', 'arm64-v8a', 'x86', 'x86_64',
+				'arm64-v8a'
+		}
 	}
 
 	lintOptions {
@@ -46,3 +60,68 @@ android {
 		}
 	}
 }
+
+task cleanAssets(type: Delete) {
+	delete 'src/main/assets'
+}
+
+task cleanIconv(type: Delete) {
+	delete 'deps/libiconv'
+}
+
+task cleanIrrlicht(type: Delete) {
+	delete 'deps/irrlicht'
+}
+
+task cleanLevelDB(type: Delete) {
+	delete 'deps/leveldb'
+}
+
+task cleanCURL(type: Delete) {
+	delete 'deps/curl'
+	delete 'deps/curl-' + curl_version
+}
+
+task cleanOpenSSL(type: Delete) {
+	delete 'deps/openssl'
+	delete 'deps/openssl-' + openssl_version
+	delete 'deps/openssl-' + openssl_version + '.tar.gz'
+}
+
+task cleanOpenAL(type: Delete) {
+	delete 'deps/openal-soft'
+}
+
+task cleanFreetype(type: Delete) {
+	delete 'deps/freetype2-android'
+}
+
+task cleanOgg(type: Delete) {
+	delete 'deps/libvorbis-libogg-android'
+}
+
+task cleanSQLite3(type: Delete) {
+	delete 'deps/sqlite-amalgamation-' + sqlite3_version
+	delete 'deps/sqlite-amalgamation-' + sqlite3_version + '.zip'
+}
+
+task cleanGMP(type: Delete) {
+	delete 'deps/gmp'
+	delete 'deps/gmp-' + gmp_version
+}
+
+task cleanAll(type: Delete, dependsOn: [clean, cleanAssets, cleanIconv,
+										cleanFreetype, cleanIrrlicht, cleanLevelDB, cleanSQLite3, cleanCURL,
+										cleanOpenSSL, cleanOpenAL, cleanOgg, cleanGMP]) {
+	delete 'deps'
+	delete 'gen'
+	delete 'libs'
+	delete 'obj'
+	delete 'bin'
+	delete 'Debug'
+	delete 'and_env'
+}
+
+dependencies {
+	implementation 'com.android.support:support-v4:28.0.0'
+}