按back键返回时，不结束当前的通话活动【生命周期】

/vendor/mediatek/proprietary/packages/apps/Dialer/java/com/android/incallui/InCallActivity.java

 @Override
  public boolean onKeyUp(int keyCode, KeyEvent event) {
    DialpadFragment dialpadFragment = getDialpadFragment();
    if (dialpadFragment != null
        && dialpadFragment.isVisible()
        && dialpadFragment.onDialerKeyUp(event)) {
      return true;
    }

    if (keyCode == KeyEvent.KEYCODE_CALL) {
      // Always consume KEYCODE_CALL to ensure the PhoneWindow won't do anything with it.
      return true;
    }

    return super.onKeyUp(keyCode, event);
  }

  @Override
  public boolean onKeyDown(int keyCode, KeyEvent event) {
    switch (keyCode) {
      case KeyEvent.KEYCODE_CALL:
        if (!InCallPresenter.getInstance().handleCallKey()) {
          LogUtil.e(
              "InCallActivity.onKeyDown",
              "InCallPresenter should always handle KEYCODE_CALL in onKeyDown");
        }
        // Always consume KEYCODE_CALL to ensure the PhoneWindow won't do anything with it.
        return true;

        // Note that KEYCODE_ENDCALL isn't handled here as the standard system-wide handling of it
        // is exactly what's needed, namely
        // (1) "hang up" if there's an active call, or
        // (2) "don't answer" if there's an incoming call.
        // (See PhoneWindowManager for implementation details.)

      case KeyEvent.KEYCODE_CAMERA:
        // Consume KEYCODE_CAMERA since it's easy to accidentally press the camera button.
        return true;

      case KeyEvent.KEYCODE_VOLUME_UP:
      case KeyEvent.KEYCODE_VOLUME_DOWN:
      case KeyEvent.KEYCODE_VOLUME_MUTE:
        // Ringer silencing handled by PhoneWindowManager.
        break;

      case KeyEvent.KEYCODE_MUTE:
        TelecomAdapter.getInstance()
            .mute(!AudioModeProvider.getInstance().getAudioState().isMuted());
        return true;
      //*/add,saving back key life activity
      case KeyEvent.KEYCODE_BACK:
           moveTaskToBack(true);
        return true;
      //*/

      case KeyEvent.KEYCODE_SLASH:
        // When verbose logging is enabled, dump the view for debugging/testing purposes.
        if (LogUtil.isVerboseEnabled()) {
          View decorView = getWindow().getDecorView();
          LogUtil.v("InCallActivity.onKeyDown", "View dump:\n%s", decorView);
          return true;
        }
        break;

      case KeyEvent.KEYCODE_EQUALS:
        break;

      default: // fall out
    }