Delay Bluetooth permission & turn-on-Bluetooth system popups on iOS

[jasaw]

PR #599 delays centralManager initialization, but instantiating FlutterBlue in flutter calls into _setLogLevelIfAvailable() which calls into isAvailable and triggers a popup anyway. This PR skips isAvailable call in the FlutterBlue constructor and allows setting of log level without initializing centralManager. The side effect of this change is, for some reason, Bluetooth permission always returns "denied" until centralManager is initialized. Similarly, Bluetooth permission under App Settings does not appear until centralManager is initialized.

[jasaw]

This PR has the unfortunate side effect where calling isOn always returns false if centralManager is initialized in the same call. This is caused by race condition of reading the Bluetooth state while the Bluetooth subsystem is still initializing.

[politebarista]

The bug is still being reproduced even in the master branch