Floss: Refactor off-hook handling to match more closely with VC app intention

When the VC app attempts to place an active call, floss simulated this
behaviour by dialing/answering call on off-hook events. This does not
match VC app's intention because the VC app want to initiate a call directly
instead of dialing call frist and then answer the call.

Remove simulated dialing/answering on off-hook events. Directly answer
incoming calls or place active calls based on the current call state on
off-hook events.

Although this commit modifies the call operation sequences, we expect no
result behaviour change to the app and end users.

Bug: 336974317
Test: Conduct the following manual tests
  pair Bluetooth Headset
  Google Meet: headset is able hang-up the call
Test: Conduct the following manual tests
  pair Bluetooth Headset
  login Ring Central and place an incoming-call
  answer the call by headset
  hangup the call by headset
Test: Conduct the following manual tests
  pair Bluetooth Headset
  login Ring Central and place an incoming-call
  answer the call on Ring Central
  hangup the call from headset
Test: atest bluetooth_test_gd
Tag: #floss
Flag: EXEMPT, no behavior change on Android; Floss-only changes

Change-Id: I7cdaa509cc2225c3d4ceacfba92d64cd97e7f8ba