Allow toggling the location that display settings are read from.

DisplayWindowSettingsProvider currently only supports reading display
window settings from a single file location. However, this doesn't
support the use case where a device may want to ship with multiple
vendor display setting experiences and allow switching between them at
runtime.

This updates DisplayWindowSettingsProvider to allow overriding the file
path that's used to read display settings. The default is still to read
from the main settings file stored at /vendor/etc/display_settings.xml
but can be changed by setting the global setting:
DEVELOPMENT_WM_DISPLAY_SETTINGS_PATH.

This also removes the flag DEVELOPMENT_IGNORE_VENDOR_DISPLAY_SETTINGS to
since ignoring vendor display settings entirely is no longer needed.

Test: atest WindowManagerSettingsTests
Test: atest DisplayWindowSettingsProviderTests

Bug: 172342357
Bug: 173540615
Change-Id: I11fd2945a4dacef80e1ab921d585c712f20ae55b