AAPT2: Fix R.java styleable + indices ordering

Make sure that Styleables are directly followed by their indices.
If not, Robolectric breaks. This is not strictly incorrect to have
an arbitrary ordering in R.java, but its easier to just support
Robolectric in this case.

Bug: 65837293
Test: make aapt2_tests

(cherry picked from commit af85c4deb667843a227d62275fe6992005f4c38d)

Change-Id: Ia59ba58427ade386d075ca9fc9eb5b53e35beca0