默认设置USB模式为MTP模式

主要修改的是UsbDeviceManager.java里handleMessage（），控制mUsbDataUnlocked 的值

@Override
public void handleMessage(Message msg) {
switch (msg.what) {
case MSG_UPDATE_STATE:
mConnected = (msg.arg1 == 1);
mConfigured = (msg.arg2 == 1);
mUsbConfigured = mConfigured;
//begin, 2017/08/24, add by zhouliaoyu, set default usb mode from only charging to mtp
if (!mConnected || mUsbDataLocked) {（避免每次mUsbDataUnlocked 都为 true, 手动设置charge only 的时候由mUsbDataLocked控制为 false ）
// When a disconnect occurs, relock access to sensitive user data
mUsbDataUnlocked = false;
} else {
mUsbDataUnlocked = true;
}
//end

。。。

。。。

case MSG_SET_USB_DATA_UNLOCKED:（手动设置模式时还是按原来的流程走，设置mUsbDataLocked来控制）
setUsbDataUnlocked(msg.arg1 == 1);
//begin, 2017/08/24, add by zhouliaoyu, set default usb mode from only charging to mtp
if(!mUsbDataUnlocked) {
mUsbDataLocked = true;
} else {
mUsbDataLocked = false;
}
//end
break;

。。。

。。。

case MSG_BOOT_COMPLETED:（开机完成的时候初始化为MTP模式）
mBootCompleted = true;
if (mCurrentAccessory != null) {
getCurrentSettings().accessoryAttached(mCurrentAccessory);
}
if (mDebuggingManager != null) {
mDebuggingManager.setAdbEnabled(mAdbEnabled);
}
//begin, 2017/08/24, add by zhouliaoyu, set default usb mode from only charging to mtp
if(mUsbDataUnlocked) {
setCurrentFunctions(UsbManager.USB_FUNCTION_MTP);
setUsbDataUnlocked(true);
}
//end
break;

。。。

。。。