Drop root privileges on startup

On rooted devices, switch to user 2000 during startup.

Copy-paste does not work as root (user 0), so switching to 2000 fixes
the issue.

Fixes #6224 <https://github.com/Genymobile/scrcpy/issues/6224>

Suggested-by: Simon Chan <1330321+yume-chan@users.noreply.github.com>

server/src/main/java/com/genymobile/scrcpy/Server.java

@@ -27,6 +27,7 @@ import com.genymobile.scrcpy.video.VideoSource;
 import android.annotation.SuppressLint;
 import android.os.Build;
 import android.os.Looper;
+import android.system.Os;
 
 import java.io.File;
 import java.io.IOException;
@@ -233,6 +234,8 @@ public final class Server {
             }
         });
 
+        dropRootPrivileges();
+
         prepareMainLooper();
 
         Options options = Options.parse(args);
@@ -273,4 +276,17 @@ public final class Server {
             // Do not print stack trace, a user-friendly error-message has already been logged
         }
     }
+
+    @SuppressWarnings("deprecation")
+    private static void dropRootPrivileges() {
+        try {
+            if (Os.getuid() == 0) {
+                // Copy-paste does not work with root user
+                // <https://github.com/Genymobile/scrcpy/issues/6224>
+                Os.setuid(2000);
+            }
+        } catch (Exception e) {
+            Ln.w("Cannot set UID", e);
+        }
+    }
 }