Remove R8-specific flags from soong config

Instead, move them to R8Wrapper. Having all of these R8-specific flags
in a single location, bundled within the R8 repo, is easier to maintain.
This should be a no-op for generated code.

Test: m + validate identical dex output
Change-Id: Ibdeecef8ca5d311d353dd623c73fab608ccd2681