BtHelper: do not clear sco clients when sco audio is disconnected

When sco audio is disconnected by BT stack, do not clear the list
of pending activation requests by apps (mScoClients). Instead, restart
sco audio if a client is still in the list.

Also restore BT SCO forced usage when speakerphone is turned off
and sco audio is connected.

Bug: 154464603
Test: repro steps in bug.
Test: manual regression tests with VoIP and cell calls

Change-Id: I54930df9da9477dfb943dc08cebd78986b6568e9