BAS: Handle disconnect right after connect

In the "connecting" state, if a disconnect request is received,
the state changed to "disconnecting" instead of "disconnected".
The "disconnecting" state always timed out because we're not
connected to the remote device yet.
This CL fixes this issue by changing the state to "disconnected"
directly.
A new test is added to confirm the behavior.

Bug: 264829420
Tag: #feature
Test: atest BatteryStateMachineTest
Change-Id: Id422e207249258642bafd28754245ac6c98f5a6e