floss: GATT: Handle concurrent write characteristic

Android and BlueZ don't allow the concurrent write characteristic and
just report `Busy`. This patch follows the behavior so Floss won't break
the previous API assumption.

Bug: 353650053
Bug: 200070162
Tag: #floss
Test: mmm packages/modules/Bluetooth
Test: SmartLock and FastPair
Test: LE mouse, battery is shown
Test: iRobot Root
Test: CTSV LE secure
Flag: EXEMPT, Floss-only changes
Change-Id: I636067892b70bac6e4a23a59f67c9e9bd6ff1cb1