Bluetooth: Audio: Remove reliance of private host values

[Thalley]

Is your enhancement proposal related to a problem? Please describe.
Bluetooth Audio can/should be seen as a user of the BT Host and should not depend on any internal APIs or header files.

Describe the solution you'd like

• Do not access bt_conn directly but use bt_conn_get_info or others where application (Bluetooth: PACS: Do not use conn struct directly #84993, Bluetooth: Host: Add API functions to determine PAST support for conn #85597)
• Do not use internal functions (Bluetooth: Host: Add API functions to determine PAST support for conn #85597, Bluetooth: Host: Make bt_addr_le_is_bonded public #85590)
• Do not use the LTK directly (https://github.com/zephyrproject-rtos/zephyr/blob/main/subsys/bluetooth/audio/csip_set_member.c#L165)
• Do not use BT settings directly (https://github.com/zephyrproject-rtos/zephyr/blob/main/subsys/bluetooth/audio/has.c#L539-L541)

Describe alternatives you've considered
N/A

Additional context
Some of the internal access can be replaced by existing public APIs.
Some of the internal access can be fixed by moving internal APIs to a public API citing LE Audio as a user.
Some of the internal access should perhaps be kept internal like accessing the LTK, but that would prevent 3rd party CSIS implementations.
Access to the BT settings is similarly also a necessity for some services, but perhaps LE Audio should have its own settings and not rely on bt_settings?