leaudio: Notify connected when subscribed for CCC

Make sure to not notify connected until all the CCC are registered and
MTU is changed on the reconnection.

Otherwise, when reconnected during streaming, stream will not be able to
setup because GATT is busy with the descritors registration and MTU
change.

Also, clears mtu_ on device disconnection

This is regression after:
4b1536c90b5  leaudio: Fix the MTU exchange request

Bug: 228351333
Test: atest BluetoothInstrumentationTests
Test: manual: Connect earbuds while streaming
Tag: #feature
Change-Id: I5b023907764e6c4552f199a8a17f02e5bddbd0de