Bluetooth: Check state correctly when client wants to bind

Use mState instead of mEnable to check Bluetooth server status.

mEnable is used to indicate whether the next action is enabling or
disabling the Bluetooth stack, whereas mState indicates the current
state.

Test: atest FrameworksServicesTests
Bug: 173941366
Change-Id: Id85494f7f40a0201061e74fbf3f08f70a632a62c