stagefright: Fix videot thumbnail retrival for wmv

We override some h/w decoders in video configure
since we know some h/w decoder does not support
certain types of video. e.g. WMV7/8.
But we did not do that when application retrieving
video frame for thumbnail causing video frame retrieved
is corrupted.

Fix it by calling FFMPEGSoftCodec::overrideComponentName
when we retrieve a single video frame for thumbnail purpose.

Change-Id: I334698c331dfd3d49bb5d8b8e9c1fe381b304179