Which Java version for Android with different minSdkVersion and targetSdkVersion

When using Gradle to compile my Android applications, how do I determine which Java version to use? For instance, some snippets of my gradle build file look like:

android {
    compileSdkVersion 22
    buildToolsVersion "22.0.1"

    // Other settings

    defaultConfig {
        applicationId "com.example"
        minSdkVersion 15
        targetSdkVersion 22

        // Other default settings
    }
}

Java SE 7 is supported for android SDK version 19 and higher, that is for KitKat and above. Given the above snippet, how do I determine whether I should use Java SE 7 or Java SE 6 for compilation and source code compatibility?