Freeform mode (Android 9.0+)

// From android.app.WindowConfiguration
final int WINDOWING_MODE_FREEFORM = 5;

ActivityOptions options = ActivityOptions.makeBasic();
options.setLaunchBounds(new Rect(0, 0, 500, 500));

try {
    Method method = ActivityOptions.class.getMethod("setLaunchWindowingMode", int.class);
    method.invoke(options, WINDOWING_MODE_FREEFORM);
} catch (Exception e) { /* handle exception */ }

// Now, call startActivity(intent, options.toBundle());