Fetch the presentation position only if the audio state is STARTED

The audio HAL will fetch the audio presentation position (for the
purpose of delay reporting if the Headset supports it) only if the
audio state is STARTED.
Otherwise, it will use the default value (e.g., the one used
when the Headset does not support delay reporting).

This avoids unnecessary transient re-opening of the HAL control path
between the Audio framework and Bluetooth native stack (e.g., during
shutdown).

Bug: 78360113
Test: Manual - A2DP streaming with Headsets that support delay reporting
Change-Id: I56f7e9373ba27606913c74fa818da40305df85d4