Process Gatt read multiple response properly

The Gatt server multi response queue is made NULL after each Gatt
server response is sent by BT stack in response to the remote Gatt
client's request. But the multi response queue is not initialized
again to process subsequent read multiple Gatt server responses.
Hence, the BT stack does not send Gatt server responses for the
subsequent read multiple requests from remote device which inturn
leads to disconnection. This change would process read multiple
responses by initializing the queue when it is null.

Some of Bluetooth certification test cases involve testing
Gatt server's capability to process and send read multiple
response.

Test case: TC_GAR_SR_BV_05_C

This change also cleans up unnecessary initialization and freeing
of multi rsp queue. This change makes sure that the stack initializes
multi rsp queue only when it is necessary(ie only when the Gatt
multiple read response is being processed).

Bug: 29011255

Change-Id: Ia3e2e1569ee16cac9c518f95501945257b9fb7fd