Sync connection state periodically when using serial

[HaoboGu]

When connecting splits using serial, there's no way to know the connection state. So after the serial disconnect, peripheral may have wrong connection state.

This PR moves the cached connection state to BLE split message writer, makes serial split driver periodically sync the connection state.

[github-actions]

PR build size

  text	   data	    bss	    dec	    hex	filename
120600	    116	  41308	 162024	  278e8	rmk-nrf52840

Diff

    FILE SIZE        VM SIZE    
 --------------  -------------- 
  +1.5%      +1  [ = ]       0    [Unmapped]
  -0.0%     -12  [ = ]       0    .strtab
  -0.0%     -13  [ = ]       0    .debug_str
  -0.0%     -24  [ = ]       0    TOTAL