LeAudio: Improve BidirectionalPair type

 - Changed get() to follow the common idiom, while copy constructor
will be called anyway if non-reference type is on the left hand side
of the assignment. There is no need for explicit reference getter.

 - It no longer requires comparison operators, which may no be defined
for the more complex types.

 - For the more complex types a single bidirectional value cannot be
 computed (e.g. a bidirectional pair of ASEs).

This allows us to wrap more complex types and simplify the code further.

Bug: 295972694
Test: atest bluetooth_le_audio_test bluetooth_le_audio_client_test  bluetooth_test_broadcaster bluetooth_test_broadcaster_state_machine
Change-Id: I35518479075732d8aa87082dd9267864161f43b3