Make ignorewarnings optional in optimize

This allows targets to opt-in to running R8 without the -ignorewarnings
flag. Ignored warnings can cause code to be optimized incorrectly, so
this makes the build safer for such targets.

Ideally ignore_warnings should default to false, but all targets need to
be fixed for that to happen. As a first step, provide a mechanism for
individual targets to ensure they do not introduce regressions.

Bug: 229727645
Bug: 180878971
Bug: 226127213
Bug: 239990030
Test: m (runs dex_test)
Change-Id: Ic0eef29598c1ee47e958da8a5048d9696165a235