Android Library and Proguard

Android Library

To build a android library project, simply change the following line in build.gradle (Module):

apply plugin: 'com.android.application'

to:

apply plugin: 'com.android.library'

To pack the library into a jar file, add these lines to build.gradle (Module):

def JarLibName="YourLibName"

task clearJar(type: Delete) {
    delete 'build/generated/' + JarLibName + '.jar'
}

task makeJar(type: Jar) {
    from zipTree(file('build/intermediates/bundles/release/classes.jar'))
    baseName = JarLibName
    destinationDir = file("build/generated")
}
makeJar.dependsOn(clearJar, build)

And run the following command in the Terminal:

./gradlew makeJar

If failed in lint process, add these to build.gradle (Module):

android {
    // ...
    lintOptions {
        abortOnError false
    }
    // ...
}

Remember to copy the jar library file to the "libs/" folder in your project.
For more infomation, refer to Packing Jar in Android Studio.

Proguard

To enable proguard and minify, add the following lines to build.gradle (Module):

android {
    // ...
    buildTypes {
        release {
            debuggable false
            minifyEnabled true
            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
        }
    }
    // ...
}

Then, modify "proguard-rules.pro" file under your module path. Keep the class you wanna expose to outside.
For instance:

-keep class yourclassname {
    public *;
}
-keep interface yourclassname$interfacename {
    *;
}
-keep class yourclassname$innerclassname {
    public static final java.lang.String SOME_CONSTANT;
    public int some_function(java.lang.String);
}

For more detail, refer to ProGuard in Android.