BT LE Android large characteristic write support

When running two Android devices where one is a central device and the other one is a peripheral LE device advertising a large writable characteristic (512 bytes), the central device appears not to be able to write to this characteristic. The MTU has been negotiated to be 512 (but due to ATT headers the whole payload won't fit one message).

It appears that the Android central device splits the package with something called "prepared write". This requires support from the peripheral side, and currently it seems the Qt bluetooth has not implemented that part.

This task is about adding that support.