cmd/gomobile: specify minSdkVersion for .aar file.

Fixes golang/go#12661.

Change-Id: I9f37159cd8090454bf2dd59126bece6647b9f3b8
Reviewed-on: https://go-review.googlesource.com/14883
Reviewed-by: David Crawshaw <crawshaw@golang.org>

cmd/gomobile/bind_androidapp.go

@@ -155,8 +155,9 @@ func buildAAR(androidDir string, pkg *build.Package) (err error) {
 	if err != nil {
 		return err
 	}
-	const manifestFmt = `<manifest xmlns:android="http://schemas.android.com/apk/res/android" package=%q />`
-	fmt.Fprintf(w, manifestFmt, "go."+pkg.Name+".gojni")
+	const manifestFmt = `<manifest xmlns:android="http://schemas.android.com/apk/res/android" package=%q>
+<uses-sdk android:minSdkVersion=%q/></manifest>`
+	fmt.Fprintf(w, manifestFmt, "go."+pkg.Name+".gojni", minAndroidAPI)
 
 	w, err = aarwcreate("proguard.txt")
 	if err != nil {