Make merged api compat checks output baseline

When intentional API breakages are made, the incompatibilities tracking
file must be updated. This makes the failing build rule output the
baseline/incompatibilities file that would have made the build rule
succeed. This file can then be used to update prebuilts/sdk.

Whilst here, also make metalava quiet so we don't need to capture
stdout.

Test: make breaking change, build compat rule, inspect output dir
Change-Id: Ia95b65548371329cd467fd3093db92b471e6986d