AAPT2: Add order attribute to groups

Require explicit ordering of groups in the configuration file to ensure
that the correct version code is set. Ordering based on a single ABI is
straight forward to ensure Play Store delivers the correct APK, but when
an APK needs more than one ABI things get messy quickly. This also goes
for screen density etc. The only thing that is easily sorted without
this attribute is android-sdk since an artifact can only reference a
single SDK.

Test: unit tests
Test: manually split an APK with update config.xml
Change-Id: I37a2b8b8a8409d6d6ff27c7142d4c8c8065a7a51