Clear local suspending flag when the BTA_AV_STOP_EVT event received

The Stack would set the pending flag when received the Stop request but
did not clear it even state became Opened.  This would cuase Stack
replied Suspended immediately to Audio HAL but kept peer at Started
state and failed to handle Start request again.

This change clears the local suspending flag when BTA_AV_STOP_EVT
received and also reset all pending status when becomes Opened so it
will take effect only when Stack is changing state from Started.

Bug: 110022304
Test: Manual: (1) Play A2DP music and switch codec
              (2) Play A2DP music and change active device

Change-Id: I4b9aca33eb7eb6c40038e2082d23eb22e5830714
Merged-In: I4b9aca33eb7eb6c40038e2082d23eb22e5830714
(cherry picked from commit e643d30249256461fd1c0a132b48c65e2eac18ba)