AAPT2: Finish support for feature splits

- Prefix the config split name generated from a feature split with the
  name of the feature split.
- Add the 'configForSplit' attribute to the <manifest> tag of a config
  split and give it the same name as the feature split it was generated
  from.
- Look for the featureSplit attribute in <manifest> and automatically
  convert it to 'split' and inject 'android:isFeatureSplit="true"'.

  Feature splits should be written like so:

  <manifest xmlns:android="http://schemas.android.com/apk/res/android"
            package="com.foo.example"
            featureSplit="feature_b">

        <uses-split android:name="feature_a" />

        ...
  </manifest>

Bug: 34703094
Test: manual
Change-Id: I01b5c4a9aa03a2d25ef1e87bc7874b57c9deede9