mapclient: use entire message handle for pushed messages

Currently, the MCE ignores the first two bytes/chars of the message
handle returned by the MSE when pushing a message to the MSE.

Message handles are assigned by the MSE and uniquely identify each
message on the MSE, so dropping the top-order bytes can be
troublesome when trying to match messages between the MCE and MSE,
e.g., when the MCE receives a SENT_SUCCESS notification from the MSE
corresponding to the message that the MCE had pushed to the MSE.

Bug: 323382399
Bug: 323381393
Bug: 288895550
Test: atest MapClientStateMachineTest
Flag: com.android.bluetooth.flags.use_entire_message_handle
Change-Id: I2f8787c17083523353ce01035edbe38433159854