Fix for Bluetooth not restarting on User Switch

The flag that keeps track of the Bluetooth adapter status(mEnable) is
not guarded correctly and could go out of sync with the adapter status.
This causes User Switch handling to not function correctly since it
checks for the status of the mEnable flag before restarting Bluetooth.
This fix queries the Bluetooth Adapter for the status instead of using
the mEnable flag.

Bug: b/37648701
Test: Tested switching users multiple times in both phone and carkit.
Also tested pairing and connection for sanity.

Change-Id: I17facecf8e1de90df11c11d5b0a184f48a0356e4