setting battery level also in hid, if it was created, yet

[grafmar]

• To be able to update the batteryLevel of the HID, it has to call the setBatteryLevel() method of the HID
• removed unused buttons member and method

[T-vK]

Hey, thanks for the contribution. Have you tested the code? I couldn't get it to display the battery level on an Android 9 phone with your code.
I tried calling bleKeyboard.setBatteryLevel(50); in both the setup function as well as in the loop function without any luck.

[grafmar]

Hi, I got interested by the video of Brian Lough (https://www.youtube.com/watch?v=4sIkW7wogrE) and was interrested if I can fix the battery level issue. I have an Android 7.1.1 phone which does not display the battery level. But with the APP "BLE Scanner" (https://play.google.com/store/apps/details?id=com.macdom.ble.blescanner&hl=de_CH) with wich I can see what is sent via bluetooth. So I tested the SW with this. I was interrested in the mouse emulation therefore I checked the SW with the ESP32-BLE-Mouse code and adapted it for the keyboard. Before the Battery level did not change, now it does. Eventhough I think it should notify and change the value automatically I had to read the value again and again to see it changing. I used a simple sketch for that which moves the mouse in a circle every 10 seconds and changes the battery level in between.

[T-vK]

Okay, I have verified this now. I'll merge this PR and hopefully create a new release later today.
This PR fixes #8.