vc: Revove pending volume control operations

Per VCS, each volume control operation should be
performed after the previous operation is responded.

As a result, a series of volume control operations
could be delayed.

This CL prevents it by removing pending volume control operations
when setting absolute volume, which is the only use case for now.

Tag: #feature
Bug: 238591929
Test: atest bluetooth_vc_test && manually keep pressing
hardware volume button with BLE headset.

Change-Id: I1aeea13a5db0a9d3ec6c816b74f76a7fd7d76080