gatt: Fix closing GATT when timeout happens on ATT bearer

When gatt_disconnect() is called it means ATT fixed channel shall be
disconnected.
Underlaying L2CAP layer will not disconnect ACL if there is any ecoc
channels opened, including EATT.

This patch, makes sure that EATT is disconnected before Fixed channel is
disconnecting.

Note: When gracefully disconnect is happening there is no issue as there
is different path to disconnect which handles EATT disconnection. Issue
happens only on different timeouts which happens on ATT.

Bug: 336298008
Bug: 361538527
Test: atest pts-bot:GATT -v
Flag: com.android.bluetooth.flags.gatt_disconnect_fix

Change-Id: Ia93c888eab2f61b21cee6736aaaafdc09a9dfe16