Offload present to a separate thread

Make Output::present() return an ftl::Future. When offloading its
HWC call, present will return a future that can be waited upon.

In CompositionEngine::present, store a vector of futures and wait
upon the results before returning. This allow's HWC's present to run in
parallel with other work; in particular, present on other displays.
Waiting here ensures that post-composition work does not start until
present has completed. Future work may defer this even later.

Reuse HwcAsyncWorker to run present on a separate thread. Add a new
variable for determining whether to run validate in parallel, since the
presence of the HwcAsyncWorker could just mean that we are offloading
present.

Read the new DisplayCapability to determine whether a display can be
offloaded to a worker thread. Only run displays in parallel if they all
have the DisplayCapability. Non HWC-enabled displays without the
capability do not prevent other displays from being offloaded. They also
run last, since they can run in parallel with HWC-enabled displays.
(The ordering is now set by SurfaceFlinger, which places physical
displays at the start of the list of outputs.)

When telling a display to offload its present call, make it only last
for a single frame. This simplifies the code while ensuring we do not
leave it enabled unnecessarily.

Leave a single present call on the main thread. This saves a thread-hop,
while still allowing it to run in parallel with other HWC work.

Only attempt to offload present if the appropriate trunk stable flag (or
debug sysprop, "debug.sf.multithreaded_present") is set.

Bug: 241285491
Bug: 259132483
Test: manual: perfetto trace
Test: libcompositionengine_test
Change-Id: Ib9d074671e32c95875ef7e0791dd95d6e595e47a