Gatt arbiter: use RwLock to allow global cleanup

The test for this code is flaky and never works on the first iteration
but works on the second attempt.
This is because the crash is cleaning the bluetooth stack and let the
arbiter be set a second time.
The proper fix is to clean the arbiter when the stack is shut down, but
being a OnceCell prevent it.
Option 1 to solve it is to no longer use a OnceCell but a optional
value that will require a lock to be shared across multiples caller.
Option 2 is to have a mutable OnceCell but this looks like an
anti-pattern

The cost of this CL is an added rwlock when doing a call on the arbiter

Bug: 287403942
Test: atest net_test_bluetooth.GattTest.GattServerBuild
Change-Id: Ibe70e51dcdade64570ff95894aad21f5bd35a109