C2SoftAomEnc: Fix onStop() implementation

Reset C2SoftAomEnc mRequestSync state properly when the stop call is made.

Whenever an IDR frame is requested, the plugin after servicing the request clears the request. This clear request call manifests as a dummy request IDR call with value of mIntf->mRequestIDR is 0.

If a request IDR is not serviced, stop call is made and codec component is reconfigured for next encode cycle then old unserviced handle idr interface handle is used to initialize plugin context, which remains unserviced through out the life of encoding.

This uncleared interface request disallows any new idr requests. So, clear the state before stopping.

Bug: 147348711
Test: atest android.mediav2.cts

Change-Id: Ic5163f492013ad0912c3c14a3305294e5a48072a