"Don't run resource union in MiniAapt" #2086 appears to break resource merging

Created by: Blisse

https://github.com/facebook/buck/pull/2086 appears to have broken our app's resource merging.

Specific breakage I noticed (the earliest breakage): we have an ids.xml with a placeholder id in a package com.my.package

The generated class appears to be found in buck-out/bin/common/my/other/package/lib__src_debug#dummy_r_dot_java__scratch/classes/com/my/package/containerconstants/R$id.class class file, but compiling the app fails with error: cannot find symbol when referenced from another package.

After reverting this change, our app compiles normally with Buck again.