Disable preDex step for jenkins builds

* Disable preDex step for jenkins builds
* Also switch from jenkins task to jenkins property
Missing space after `if`.

Btw, is this enough or do we need the more sophisticated version suggested by the [Android devs](http://tools.android.com/tech-docs/new-build-system/tips#TOC-Improving-Build-Server-performance.)? (I like their parameter name btw).
```Groovy
project.ext.preDexLibs = !project.hasProperty('disablePreDex')

subprojects {
    project.plugins.whenPluginAdded { plugin ->
        if ("com.android.build.gradle.AppPlugin".equals(plugin.class.name)) {
            project.android.dexOptions.preDexLibraries = rootProject.ext.preDexLibs
        } else if ("com.android.build.gradle.LibraryPlugin".equals(plugin.class.name)) {
            project.android.dexOptions.preDexLibraries = rootProject.ext.preDexLibs
        }
    }
}
```