Copy an AVRC packet before sending a response back

Don't reuse AVRC buffers for responding back. Apparently,
in the AVRC packets we transmit the offset with the payload must
be at least 15 octets. If the original buffer is not large enough,
this results in memory corruption.

Also, use an explicit check for 'bt_rc_ctrl_callbacks' as a workaround
until the upper layer does the right thing with the callbacks registration.

Bug: 22006014

Change-Id: I28c248d1580bdddbda76298d19faadf8985187fc