proposal: x/mobile: bind support list of types to not export

[xcolwell]

Proposal Details

gomobile bind currently tries to export all exported types. There are cases like RPC and encoding where some types might be exported by the package, but should not be exported by gomobile (due to language feature support). In these cases, it would be clearer for usage to have a list of unexported types, or a compiler annotation for types, than to see random warnings in the output about unsupported types.

[ianlancetaylor]

CC @golang/android @golang/ios