Remove dalvik.vm.madvise.artfile.size default

The default for this property is now defined in the runtime, in
anticipation of making it 0 (disabled). Defining the legacy default
in the runtime, and guarding with a flag, let's us ramp with A/B
comparative results.

Flag: android.os.disable_madvise_artfile_default
Test: m
Bug: 382110550
Change-Id: I9aaa92d3efd7653f345a667d28fd8172a9dbf741