Different string.xml files according to app Build Variants

Yes it's possible!

In your build.gradle you can add something like this:

buildTypes {
    release {
        minifyEnabled false
        proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.txt'
        signingConfig signingConfigs.release
    }
    debug {
        applicationIdSuffix ".debug"
        minifyEnabled false
        debuggable true
        signingConfig signingConfigs.release
    }
}

Then in your src folder in your project you should have a folder called main, add one next to it called debug. Den as long as you are building your debug flavour any resources in your debug folder will replace those in main which is the release folder.

Should look like this:

src/
    main/
        java/ -- all your java code
        res/
            ...
            values/
                strings.xml
    debug/
        res/
            ...
            values/
                strings.xml

EDIT: The approaches from the two other answers works fine as well. But if you have a lot of strings keeping them as xml might be easier to handle.

It possible via your build.gradle file

buildTypes {
    release {
        resValue "string", "my_string", "some release value"
    }
    debug {
        resValue "string", "my_string", "some debug value"
    }
}

Then you can just use this value like @string/my_string where you want