cmd/gomobile: add generated package names to proguard.txt

No test as I couldn't figure out how to persuade gradle test to
strip classes.

Fixes golang/go#30251

Change-Id: I7dca9169a4a8eb31b981ad49047a6bdfa6f6efd7
Reviewed-on: https://go-review.googlesource.com/c/164798
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
diff --git a/cmd/gomobile/bind_androidapp.go b/cmd/gomobile/bind_androidapp.go
index 149fab4..3b01adc 100644
--- a/cmd/gomobile/bind_androidapp.go
+++ b/cmd/gomobile/bind_androidapp.go
@@ -155,6 +155,13 @@
 		return err
 	}
 	fmt.Fprintln(w, `-keep class go.** { *; }`)
+	if bindJavaPkg != "" {
+		fmt.Fprintln(w, `-keep class `+bindJavaPkg+`.** { *; }`)
+	} else {
+		for _, p := range pkgs {
+			fmt.Fprintln(w, `-keep class `+p.Name+`.** { *; }`)
+		}
+	}
 
 	w, err = aarwcreate("classes.jar")
 	if err != nil {