How to generate on the fly Java source code to build with Gradle in Android Studio?

You could just write the generated file to the $buildDir/generatedJava then add that directory as a source folder. Then make the task javaCompile depend on your task that generates the source. Something like this could be used inside a build.gradle

def outputJavaFile = new File("$buildDir.absolutePath/generatedJava", 'MyEnum.java')
task generateSources(type: Exec) {
    def source = $/
package com.example;
public enum Something {
   One,
   Two
}
/$

    if (!outputJavaFile.parentFile.exists()) {
        outputJavaFile.parentFile.mkdirs()
    }
    outputJavaFile.withWriter {
        it << source
    }
}

compileJava.dependsOn outputJavaFile

sourceSets {
    main {
        java {
            srcDirs 'src/main/java', outputJavaFile.absolutePath
        }
    }
}

// if the goal is to generate the source from a script then just call the script
// inside the Exec closure
task shellScriptToGenerateSources(type: Exec) {
    commandLine 'myShellScript.sh'
}
// then make compileJava depend on the task that runs the script
compileJava.dependsOn shellScriptToGenerateSources

Edit: To apply the same logic to your updated build.gradle file might look something like this

apply plugin: 'com.android.model.application'

// define an output folder for our generated .java files
def GENERATED_JAVA_OUTPUT = "$buildDir/generatedJava"
// if the goal is to generate the source from a script then just call the script
// inside the Exec closure
task shellScriptToGenerateSources(type: Exec) {
    commandLine 'myShellScript.sh'
}
// then make compileJava depend on the task that runs the script
compileJava.dependsOn shellScriptToGenerateSources

model {

    compileOptions.with {
        sourceCompatibility = JavaVersion.VERSION_1_7
        targetCompatibility = JavaVersion.VERSION_1_7
    }

    android {
        compileSdkVersion = 23
        buildToolsVersion = "23.0.2"

        defaultConfig.with {
            applicationId = "..."
            minSdkVersion.apiLevel = 18
            targetSdkVersion.apiLevel = 23
        }

        defaultConfig.multiDexEnabled = true
    }

    android.ndk {
        platformVersion = "18"
        moduleName = "jni"
        ...
    }

    android.buildTypes {
        release {
            minifyEnabled = false
            proguardFiles.add(file('proguard-rules.txt'))
            ndk.with {
                CFlags.add("-Werror")
                cppFlags.add("-Werror")
            }
        }
    }

    android.productFlavors {
        create("arm7") {
            ndk.abiFilters.add("armeabi-v7a")
        }
    }

    // let android know that our java sources shoudl also consider the generated java for the compiler
    android.sourceSet {
        main {
            java {
                srcDir(GENERATED_JAVA_OUTPUT)
            }
        }
    }
}

repositories {
    ...
}

dependencies {
    ...
}

apply plugin: 'com.google.gms.google-services'