java: format strings containing integers in ROOT locale.

We use strings to pass structured data from the JVM to Go.
In a locale using Indian-Arabic numerals: ۰ ۱ ۲ ۳ ۴ ۵ ۶ ۷ ۸ ۹
the Java code will format decimal strings using Indian-Arabic
glyphs.

Go doesn't get a locale set automatically by the Android
runtime, so it always parses strings in a default en-US
`unable to parse "lo ١ ٦٥٥٣٦ true false true false false |": expected integer`

Make the Java code format using the ROOT locale. These strings
are purely internal to pass between the two runtimes, they are
not shown to the user.

Fixes https://github.com/tailscale/tailscale/issues/4156

Signed-off-by: Denton Gentry <dgentry@tailscale.com>

android/src/main/java/com/tailscale/ipn/App.java

@@ -54,6 +54,7 @@ import java.security.GeneralSecurityException;
 import java.util.ArrayList;
 import java.util.Collections;
 import java.util.List;
+import java.util.Locale;
 
 import androidx.core.app.NotificationCompat;
 import androidx.core.app.NotificationManagerCompat;
@@ -373,7 +374,7 @@ public class App extends Application {
                 try {
                     // Android doesn't have a supportsBroadcast() but the Go net.Interface wants
                     // one, so we say the interface has broadcast if it has multicast.
-                    sb.append(String.format("%s %d %d %b %b %b %b %b |", nif.getName(),
+                    sb.append(String.format(java.util.Locale.ROOT, "%s %d %d %b %b %b %b %b |", nif.getName(),
                                    nif.getIndex(), nif.getMTU(), nif.isUp(), nif.supportsMulticast(),
                                    nif.isLoopback(), nif.isPointToPoint(), nif.supportsMulticast()));
 
@@ -381,7 +382,7 @@ public class App extends Application {
                         // InterfaceAddress == hostname + "/" + IP
                         String[] parts = ia.toString().split("/", 0);
                         if (parts.length > 1) {
-                            sb.append(String.format("%s/%d ", parts[1], ia.getNetworkPrefixLength()));
+                            sb.append(String.format(java.util.Locale.ROOT, "%s/%d ", parts[1], ia.getNetworkPrefixLength()));
                         }
                     }
                 } catch (Exception e) {

android/src/main/java/com/tailscale/ipn/DnsConfig.java

@@ -19,6 +19,7 @@ import java.net.InetAddress;
 
 import java.util.ArrayList;
 import java.util.List;
+import java.util.Locale;
 
 // Tailscale DNS Config retrieval
 //
@@ -229,7 +230,7 @@ public class DnsConfig {
 
 
 	String intToInetString(int hostAddress) {
-		return String.format("%d.%d.%d.%d",
+		return String.format(java.util.Locale.ROOT, "%d.%d.%d.%d",
 			(byte)(0xff & hostAddress),
 			(byte)(0xff & (hostAddress >> 8)),
 			(byte)(0xff & (hostAddress >> 16)),