AAPT2: Generate R.txt

In order to support a staged rollout of support for AAPT2,
libraries being built the old way (merged into a single resource
directory) still need to make use of the generated R.txt AAPT
emitted. Do the same as AAPT.

Test: manual
Change-Id: Iaac1e824ddbd67e4efbab7692cddc1e4aa052f5a