Put javac, d8 and r8 rules into RBE ninja pool, take two

I60f7859f3657fc1eff0daf0e0a1b0f9fe63551b6 attempted to put the javac,
d8 and r8 rules into the RBE ninja pool so they could run 500 jobs
in parallel, but rbe.mk is not parsed during the main build phase
and the pool continued to be unset.  Move the pool configuration
into config.mk.

Bug: 228603472
Test: manual
Change-Id: I56377244e1c496034576ba5988393d88c2c5b09f