C2SoftOpusEnc: Fix mono encoding

Number of coupled streams was wrongly initialized during codec create
which is now fixed based on number of channels

Also, limit number of channels supported to 2

Bug: 132798253
Test: atest android.media.cts.EncoderTest#testOpusEncoders

Change-Id: Ib0705c805c5518f94765a605a928f002df21cd5f