WMS/ViewRoot: Prepare for Async BLAST Sync [1/N]

We would like to make relayoutWindow a non-blocking call,
and of course why not, who likes blocking. However it functions
as a critical path of BLASTSync. To understand why examine the
Guarantee described in BLASTSync.md.

In order to implement this guarantee we need to know
“Which frame has the client finished drawing” when it calls
finishDrawing. The current answer to this question is
“The frame reflecting the state observed in the last call
to relayoutWindow”. The comments on mPending and mCurrentDrawHandlers
also have a lot more context on how this works currently. Since
relayoutWindow has a critical section, it also ensures that changes
to syncable state and preparation of sync will be observed atomically
(since they are both observed over relayoutWindow, which is always
called before any frame drawing updated syncable state).

We design a new protocol described in BLASTSync.md, which uses a seqId
to track which call to finishDrawing reflects which syncable state.
We implement this seqId based system. Unfortunately the WindowManager
doesn’t quite conform to the requirements specified by the protocol,
and a follow up CL ensures that syncable state changes will
always be sent with the seqId.

This CL simply adds the protocol specification and makes some required
interface changes. It can be seen to be a no-op.

Bug: 161810301
Bug: 175861051
Bug: 175861127
Bug: 200285149
Change-Id: If2dea07121fe7de5d2524c5b63678dddf955d4b7