dashboard: enable android-amd64-emu and android-386-emu builders

Updates golang/go#23824

Change-Id: I24b590dab6719d014472da3d7850fc9a1b3ef4cc
Reviewed-on: https://go-review.googlesource.com/c/164380
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

dashboard/builders.go

@@ -1749,8 +1749,6 @@ func init() {
 		Name:     "android-386-emu",
 		HostType: "host-android-amd64-emu", // same amd64 host is used for 386 builder
 		Notes:    "Android emulator on GCE",
-		TryOnly:  true, // but not in trybot set for now
-		tryBot:   nil,
 		env: []string{
 			"GOARCH=386",
 			"GOOS=android",
@@ -1763,8 +1761,7 @@ func init() {
 		Name:     "android-amd64-emu",
 		HostType: "host-android-amd64-emu",
 		Notes:    "Android emulator on GCE",
-		TryOnly:  true, // but not in trybot set for now
-		tryBot:   nil,
+		tryBot:   nil, // not yet; TODO: collect timing, deciding test sharding size
 		env: []string{
 			"GOARCH=amd64",
 			"GOOS=android",