C2SoftOpusEnc: Fix higher bitrate encoding at lower sample rates

Output buffer allocated was not large enough to hold higher bitrate
outputs for lower sample rates (eg: 512kbps at 8000 Hz)

Also pass remaining bytes in the allocated output buffer correctly
to encoder.

Bug: 134087860
Test: Apply CTS cl mentioned in the bug
Test: atest android.media.cts.EncoderTest#testOpusEncoders

Change-Id: I01a75cb76d15b733690e35fa107ec973668d19bb