Don't change the framebuffer target until we render a new one

Continuing to send the last-rendered framebuffer to HWC on subsequent
frames allows the HWC to read partially-composed regions that haven't
changed, instead of re-composing from scratch.

Bug: 11573910
Change-Id: I8829877d2a06001f1e1b3f168cbba71c7b217b2d