x/mobile: gobind drops package function comments

[i9]

What version of Go are you using (go version)?

$ go version
go version go1.11.10 darwin/amd64

Does this issue reproduce with the latest release?

yes

What did you do?

gomobile bind to generate Android .aar and sources.jar

What did you expect to see?

Package level function comments are kept as javadoc

What did you see instead?

Package level function comments are ignored in generated java file

[i9]

identify the cause at bind/genjava.go

https://go-review.googlesource.com/c/mobile/+/181038 for review

[gopherbot]

Change https://golang.org/cl/181038 mentions this issue: bind/genjava: keep package function comments in java file