A2DP: Reset the HAL pending command flag while clean up

In order to switch the audio HAL between software and offloading
encoders, there is a static flag shared between two HAL interfaces, and
is used to know whether the stack is still handling a previous task.
If audio HAL asked to start or suspend the playback while disabling
Bluetooth, the flag was assigned, but might not reset after A2DP clean
up. Since this is a static variable, it would be a worng value after
re-enabled, and should be reset while clean up or HAL interfaces'
initialization.

Bug: 147394596
Test: Turn off Bluetooth when A2DP playback
Change-Id: If14235f9cc302895049404a562ca0a27f416a3f2