Tune the GC ergnomics for the read barrier config.

Add 1.0 to the heap growth multiplier for the read barrier config, which
reduces the GC frequency down to roughly the same as CMS in one of the
jank tests.

Bug: 29517059
Bug: 12687968
Test: ART run-tests.
Change-Id: I1302a2f17e862f152d2f92bc06a65c9e6defcba0