Update Android build scripts. Remove unnecessary project files

pkg/android/app/.gitignore

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+/build

pkg/android/app/build.gradle

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+apply plugin: 'com.android.library'
+
+android {
+    compileSdkVersion 28
+    defaultConfig {
+        minSdkVersion 14
+        targetSdkVersion 23
+        versionCode 1
+        versionName "1.0"
+        testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"
+        externalNativeBuild {
+            cmake {
+                cppFlags ""
+            }
+        }
+        ndk {
+            // Tells Gradle to build outputs for the following ABIs and package
+            // them into your APK.
+            abiFilters "armeabi-v7a", "arm64-v8a", "x86", "x86_64"
+        }
+    }
+    buildTypes {
+        release {
+            minifyEnabled false
+            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
+        }
+    }
+    externalNativeBuild {
+        cmake {
+            path "../../../CMakeLists.txt"
+        }
+    }
+}
+
+dependencies {
+    implementation fileTree(dir: 'libs', include: ['*.jar'])
+    implementation 'com.android.support:appcompat-v7:28.0.0-alpha3'
+    implementation 'com.android.support.constraint:constraint-layout:1.1.2'
+    testImplementation 'junit:junit:4.12'
+    androidTestImplementation 'com.android.support.test:runner:1.0.2'
+    androidTestImplementation 'com.android.support.test.espresso:espresso-core:3.0.2'
+}

pkg/android/app/proguard-rules.pro

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+# Add project specific ProGuard rules here.
+# You can control the set of applied configuration files using the
+# proguardFiles setting in build.gradle.
+#
+# For more details, see
+#   http://developer.android.com/guide/developing/tools/proguard.html
+
+# If your project uses WebView with JS, uncomment the following
+# and specify the fully qualified class name to the JavaScript interface
+# class:
+#-keepclassmembers class fqcn.of.javascript.interface.for.webview {
+#   public *;
+#}
+
+# Uncomment this to preserve the line number information for
+# debugging stack traces.
+#-keepattributes SourceFile,LineNumberTable
+
+# If you keep the line number information, uncomment this to
+# hide the original source file name.
+#-renamesourcefileattribute SourceFile

pkg/android/app/src/main/AndroidManifest.xml

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+<?xml version="1.0" encoding="utf-8"?>
+<manifest xmlns:android="http://schemas.android.com/apk/res/android"
+    package="com.example.zerotier">
+
+    <uses-permission android:name="android.permission.INTERNET" />
+    <uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />
+
+</manifest>
+

pkg/android/app/src/main/java/com/example/zerotier/MainActivity.java

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+

pkg/android/app/src/main/java/com/zerotier/libzt/README

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+Before build, Java wrapper sources will be copied here