Suppress binding warnings.

[jpobst]

Suppress the warnings emitted by generator by default. These 19K+ warnings are clearly not things that we intend to take action on, and hide warnings we might want to take action on.

This is controlled via the $(_AndroidIgnoreGeneratorWarnings) property.

If seeing these warnings is desired on a local build, either:

• Change $(_AndroidIgnoreGeneratorWarnings) to false in the Directory.Build.props file.
• Pass it as a build property:
  • dotnet build -p:_AndroidIgnoreGeneratorWarnings=false

Also fix some IsTrimmable warnings by not enabling it on netstandard2.0 projects.