原生设置里，系统-语言和输入法-实体键盘-使用屏幕键盘 选项，

关闭时，外接物理键盘，如USB键盘，输入时不会弹出软键盘。
打开时，外接物理键盘，如USB键盘，输入时会弹出软键盘。

这个选项默认是关闭的。

改为打开，如有需要，可通过 overlay 的方法修改。

--- a/frameworks/base/packages/SettingsProvider/res/values/defaults.xml
+++ b/frameworks/base/packages/SettingsProvider/res/values/defaults.xml
@@ -38,4 +38,7 @@
     <!-- Decrease animation duration. -->
     <fraction name="def_window_animation_scale">50%</fraction>
     <fraction name="def_window_transition_scale">50%</fraction>
+
+    <!-- Default for Settings.Secure.SHOW_IME_WITH_HARD_KEYBOARD -->
+    <bool name="def_show_ime_with_hard_keyboard">true</bool>
 </resources>

Settings 源码在 ./packages/apps/Settings/src/com/android/settings/inputmethod/PhysicalKeyboardFragment.java ，

    @Override
    public void onResume() {
        super.onResume();
        mLastHardKeyboards.clear();
        scheduleUpdateHardKeyboards();
        mIm.registerInputDeviceListener(this, null);
        mShowVirtualKeyboardSwitch.setOnPreferenceChangeListener(
                mShowVirtualKeyboardSwitchPreferenceChangeListener);
        registerShowVirtualKeyboardSettingsObserver();
    }

   private void registerShowVirtualKeyboardSettingsObserver() {
        unregisterShowVirtualKeyboardSettingsObserver();
        getActivity().getContentResolver().registerContentObserver(
                Secure.getUriFor(Secure.SHOW_IME_WITH_HARD_KEYBOARD),
                false,
                mContentObserver,
                UserHandle.myUserId());
        updateShowVirtualKeyboardSwitch();
    }

    private void updateShowVirtualKeyboardSwitch() {
        mShowVirtualKeyboardSwitch.setChecked(
                Secure.getInt(getContentResolver(), Secure.SHOW_IME_WITH_HARD_KEYBOARD, 0) != 0);
    }
    
    private final OnPreferenceChangeListener mShowVirtualKeyboardSwitchPreferenceChangeListener =
            (preference, newValue) -> {
                Secure.putInt(getContentResolver(), Secure.SHOW_IME_WITH_HARD_KEYBOARD,
                        ((Boolean) newValue) ? 1 : 0);
                return true;
            };