end2end: Implement a fake IComposerClient [4/N]

Implements a minimal but function ComposerClient which responds to all
the initial SurfaceFlinger queries needed to allow fake physical
displays to be connected and used.

The implementation code is split into two parts. First is a series of
special delegating helper implementations, located under the hwc3/delegators
directory. These implement the full interface by delegating to a
templated handler function. This allows the default behavior for the
full interface to be specified by just writing a single function,
whether that behavior is to return an error for all calls, or to do
something else.

This is then used as a base class by the actual implementation which
focuses on overriding only the interface functions which need special
handling.

The existing files are shuffled around a bit, as the delegators will be
reused by a subsequent CL to define a intercepting/forwarding observer
implementation, and this will be distinct from the "fake" implementation
as it will eventually be used to observe the interactions between SF and
the drm_hwcomposer real implementation for use by the test.

This change also introduces a local SharedMutex helper, which wraps
`std::shared_mutex` and enforces the correct locking for the two types
of access (exclusive or shared).

The shared mutex allows the display changes to be made by the test
potentially at the same time as SF is making calls to query the display
properties.

With the fake implementation correctly configured for SurfaceFlinger to
use, SF now boots and enters a usable state. The test can now control
SF through the front-end interfaces, though there are other changes
still needed to make presentation work.

Flag: TEST_ONLY
Bug: 372735083
Test: atest surfaceflinger_end2end_tests

Change-Id: Ic67ed4ed0627649c70910cb20643bfa0c7b75208