[HWUI] fix texture cache can't be removed for recycled bitmap

Symptom: recycled bitmap may show in another image view
Root Cause: hwui texture cache didn't remove the old cache that the pixelRef has been released,
            so mmap may return the same address to fit new pixel buffer
            and TextureCache will hit the old texture.

Solution: remove setPixel(NULL, NULL) code to prevent TextureCache::clearGarbage can't find pixelRef key value.

Change-Id: If4d4f4e8e42c7bf1ef9194cc944019e6e8259cdd