hoststubgen: Slightly refined stats.

To give more accurate statistics, focus only on public non-abstract
methods.  Shift measurement back to `BaseAdapter`, but only collect
stats when operating in the `impl` mode; this ensures we're early
enough to accurately count `substitute-from` methods.

Also ignore common generated code like Protos, StatsLog, and
LogTags, as we'll eventually be handling them all in a more
consistent way.

Bug: 324417456
Test: atest FrameworksCoreTestsRavenwood
Change-Id: I07e0ea4107b27dbfa241c48c9f05def445b78cfe