Update Exopackage article with better Gradle numbers

Summary:
We were given some tips for improving Gradle's build performance, and we
got a request for more details about our experimental setup.

Add updated numbers for Gradle (they are a little better), and a blurb
about our setup.

Test Plan: Looked at the page in soyweb-local.
Revert "Allow read only commands to be executed concurrently."

Summary:
This reverts commit 10ff147e50bc83ea75d456fbff7b5066a7641d6f.
[super easy] Fix a style nit.

Test Plan: None.
Refactor MergeAndroidResourcesStep$Resource to a high level class.

Summary:
I am working on changing how we generate the uber R.java file. I am going to
use this class to represent an R.txt file entry, hence the name change.

Test Plan: buck test --all