Actually use --extra-r-text-files in process_resources.py

Generate extra packages' R.java files only containing the resources
listed in their R.txt files. This significantly reduces the number of
DEX field IDs taken up by resources.

BUG=480036
R=cjhopman

Review URL: https://codereview.chromium.org/1104703003

Cr-Commit-Position: refs/heads/master@{#328565}
7 files changed