floss: Change manager GetState to GetAdapterEnabled

btmanagerd has 4 internal states (on, off, turning on, turning off) but
these aren't useful to the manager clients. Thus, replace the GetState
api with a GetAdapterEnabled api which returns True only when an adapter
is fully on.

Bug: 194420996
Tag: #floss
Test: Verified on ChromeOS device
Change-Id: I69391699c64590cb93cc028100b9003b2f96d181