自动混淆Release版本

buildTypes {
    release {
        minifyEnabled true
        useProguard true
        proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
        signingConfig signingConfigs.release
        shrinkResources true
        debuggable false
        jniDebuggable false
        renderscriptDebuggable false
        zipAlignEnabled true
        pseudoLocalesEnabled false
    }
    debug {
        signingConfig signingConfigs.debug
    }

proguard-rules.pro文件，如果用了okhttp控件，需要用下面的规则文件，另外一半需要把安卓系统库的混淆去掉，否则可能程序运行时会崩溃，记住，混淆后，一定要测试一下所有功能：

-renamesourcefileattribute SourceFile
-dontwarn okio.**
-dontwarn com.squareup.okhttp3.**
-keep class com.squareup.okhttp3.** { *; }
-keep interface com.squareup.okhttp3.* { *; }
-dontwarn javax.annotation.Nullable
-dontwarn javax.annotation.ParametersAreNonnullByDefault

-dontwarn android.support.design.**
-keep class android.support.design.** { *; }
-keep interface android.support.design.** { *; }
-keep public class android.support.design.R$* { *; }

-keep public class android.support.v7.widget.** { *; }
-keep public class android.support.v7.internal.widget.** { *; }
-keep public class android.support.v7.internal.view.menu.** { *; }

-keep public class * extends android.support.v4.view.ActionProvider {
    public <init>(android.content.Context);
}

-keep interface android.support.v4.** { *; }
-keep interface android.support.v7.** { *; }
-keep class android.support.** { *; }