[flexiglass] Overrides some SysUiState flags.

SysUiState is a central holder of state flags for a lot of System UI, it
models state as a collection of "flags" where each flag has a unique
integer ID and either a true or false value. The internal modeling uses
bitwise operations to keep that state, likely for legacy performance
reasons that have since become more Cargo Cult than reality.

While it's true that Flexiglass' SceneContainerStartable already
actively hydrated SysUiState flags related to current scene and scene
switching (for example: shade visible, bouncher showing, etc.), there
are other non-Flexiglass classes who also write values for the same
flags and, since those classes don't read their own state from
Flexiglass, they often provide incorrect state to SysUiState, leading to
the current bug related to back navigation from the bouncer.

While the full long-term approach is to eliminate SysUiState and convert
its state to a collection of repositories and interactors, such a change
would be large and risky. This is captured in b/322510930.

Instead, a shorter-term approach is used by this CL. We introduce a
"plugin" class that knows about Flexiglass and provides "overrides" for
flag values. When any other class in the codebase attempts to write
values for certain flags in SysUiState, the plugin is consulted to check
if Flexiglass has a different value and, if so, the different value is
maintained.

Test: manually verified, together with the followup CL on this chain,
that the bouncer scene's back gesture and button (in 3-button nav mode)
actually works as intended - before this CL, there was no back
affordance at all.
Test: unit tests updated.
Bug: 322518343
Bug: 303131743
Flag: ACONFIG com.android.systemui.scene_container DEVELOPMENT

Change-Id: I3bf55f10f98fe61dfac689959b6655fecc3c3c62