Goal: enable rendering feedback loop detection for Manhattan.

Making getFramebuffer private prevents direct access to the Framebuffer
internals. Hiding access lets the FramebufferVk class switch up its
implementation to support different VkFramebuffers for feedback loop
cases.

Also this is a general code cleanup.

Bug: angleproject:4490
Change-Id: Ib4f5e34d476ccd8fc43a1adb031933ab347ca7b7
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2109333Reviewed-by: Cody Northrop <cnorthrop@google.com>
Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com>
Commit-Queue: Jamie Madill <jmadill@chromium.org>