Floss: add sysprop |enable_le_audio_only|

This is a temporary flag used for prototyping and development.

When this flag is enabled, it will disable classic profiles and only
attempt connection to LEA profiles.

We expect to have dual mode in the near future as the user-facing flag.

Bug: 317682584
Test: m Bluetooth
Flag: EXEMPT floss only changes
Change-Id: I67dc918a54d16c06720034d57b037f5df3c0347f