A2DP: Wait for previous AVDTP instance to be clean up before re-enabling

Because more connections spend more time to disconnect, the stack may
still be busy at disabling AVDTP, if we re-enable the A2dpService
quickly. This change will defer BTA_AV_API_ENABLE and BTA_AV_API_REG
events if the stack is still disabling the previous instance, so gives
the stack half more seconds to handle its previous task.

Bug: 135655859
Test: manually
Change-Id: I8e700ce195aa634819ea1ad8cf5892cc35669e10