Gate controls controllers with system feature flag

Instantiation of the Controls Controllers is now flagged behind
PackageManager.FEATURE_CONTROLS. This is done in the following way:
* The class ControlsComponent has accessors to the top level
controllers (as Optional), but they will be Optional.empty() if the
feature is disabled.
* GlobalActionsDialog injects a ControlsComponent and works with the
optionals.
* ControlsRequestReceiver checks this flag and returns immediately if
it's false.

In this way, the whole controls subsystem is only instantiated if the
feature is supported.

Bug: 157244528
Test: manual (enable and disable the flag).
Test: atest ControlsComponentTest ControlsRequestReceiverTest
Change-Id: I05419bb28289afbf5813791cfc72c54a123f4c87