[Drop-IN] Expose voiceInstructionsPlayer in drop-in

[korshaknn]

Description

need to expose as a flow because MapboxAudioGuidanceVoice is created in MapboxAudioGuidance when some flows are combined, so we have some delay. If we don't use a flow, and expose a player directly, it will be null until combine is triggered ^^, i.e. it will be null if we use it in activity::onCreate