Add camera extension fallback toggle

Android camera extensions
(https://source.android.com/docs/core/camera/camerax-vendor-extensions)
will be able to use SW fallback implementations
on devices that do not ship the corresponding
the device specific functionality.
Since the SW fallback will be using a new data
path, it was suggested that users must be able to
control and enable/disable the SW fallback
via the Settings app.

Bug: 297083874
Test: atest packages/apps/Settings/tests/unit/src/com/android/settings/privacy/CameraExtensionsFallbackPreferenceControllerTest.java

Change-Id: I1b97777babe1c9f4ea4f2f6ee3d8251fea11146e