bta: le_audio: Fix the default struct ase cis_conn_handle

The 0 is valid connection handle, thus the default CIS Connection
handle shall be set to value out of range 0x0000 to 0x0EFF.
This introduces kInvalidCisConnHandle and used this value by
default, as uninitialized value.

Bug: 357472821
Flag: Exempt; trivial change covered with unit tests
Test: atest bluetooth_le_audio_test
Test: atest bluetooth_le_audio_client_test
Change-Id: Ie89d74bd5757c49e5ec53395ff952c4d825963fd