CD settings: clean up null handling

Keep a non-null injector in the ExternalDisplayPreferenceFragment so
that we don't need to check it whenever it is used.

This makes the wrappers in ExternalDisplaySettingsConfiguration
redundant, since their main purpose was to check for a null injector.

Stop passing a Context object around several methods. Instead, just use
requireContext() whenever we need a context. Only a fraction of the
methods actually need to use the context directly. getPrefContext() does
not work because it cannot be overridden in tests (it is final) and
requires PreferenceFragmentCompat.mPreferenceManager to be set.

Bug: b/352648432
Flag: EXEMPT refactor
Test: ExternalDisplayUpdaterTest
Test: ExternalDisplayPreferenceFragmentTest
Change-Id: I5aa3703c2646fc12fb9b77661fc2f3c35388f61b