audioservice: fix not able to disconnect VoIP call

startBluetoothSco() is not safe enough from subseqent method call which
was found on the Wechat. For instance, VoIP call will not be terminated
properly in the following call flows:
startBluetoothSco() (-> incCount) ->
startBluetoothSco() (-> incCount) ->
stopBluetoothSco() (-> decCount) -> at this point, it cannot reached to
disconnectBluetoothScoAudioHelper() due to mStartCount is not 0 from
requestScoState()

To fix this, mStartCount was removed because it's usage is unclear.
Also incCount, decCount was removed because it makes difficult to
understand what the purpose of those functions from their name.

Bug: 150312620
Test: manual test cell and VoIP calls with BT SCO HS.

Change-Id: Iabc44795b8a87c10b00d84c67de9938bef385841
Signed-off-by: slki.shin <slki.shin@samsung.com>