Enable same-file policy for R8 optimizations

While this does increase dex size for optimized targets by limiting the
scope of class merging, it also
  1) improves actionability of unretraced stack frames
  2) improves accountability for size increases
  3) tends to decrease compiled code sizes for impacted targets
  4) marginally (~1%) decreases RSS

Improvements in tooling should mitigate (1) and (2), and more targeted
use of @NeverInline can mitigate (3). Until such time, the pros seem to
outweigh the cons, so we enable the policy as a temporary measure.

Bug: 264916934
Test: m
Change-Id: Id63a17955692b75b16c42d9c85b55bae409c0da4