SoftAVCEnc: Modified the code for runtime change in params to be generic

mBitrateUpdated and mKeyFrameRequested are removed and instead mUpdateFlag
with one bit for each param, will be used to configure the codec in runtime.
This change will make it cleaner to configure more parameters in run-time

Change-Id: I935827b54b0de469fa3c83237cccd4b5e4dcedd6