Using the number of commits in the HEAD to auto-increment your android version count.

build.gradle

//Counts the number of commits on the HEAD of the repo.
def headCommitCount() {
    def cmd = "git rev-list HEAD --count"
    return cmd.execute().text.toInteger()
}

android {
    defaultConfig {
        versionCode headCommitCount() //Auto-increment the version based on how many commits have occurred.
        versionName "x.x.x (" + versionCode + ")"
    }
}