Enable stack automatically when btadapterd starts

Enable and disable is already handled by btmanagerd by starting and
exiting btadapterd daemon, so btadapterd does not need to expose enable
and disable interfaces but making assumption that it needs to enable the
stack when it starts and disable when exits.

Bug: 186492781
Tag: #floss
Test: manual - Run btadapterd and observe that the stack is enabled.

Change-Id: I614ec12df1d41043ee98c31569557e2c45f52556