Hearing Aid: leave some radio time for other devices

We should send maximum two packets in connection event, and they
should take less than 3.75ms with acknowledgements.
Leaving some radio time should make us able to keep third LE connection
to some less demanding devices.

Bug: 78364114
Bug: 69623109
Test: stream music through Hearing Aid and connect to some LE device
Change-Id: Ie97b9db456a957c11438208448c562435b1b0222