Do not build remoteauth in udc branches

Remotauth is not compatible with U, so building it in udc-based branches
is not necessary, and it would be hard to backport dependencies as
needed.

This change causes fallback sources to be built instead of
remoteauth/service and remoteauth/framework: stubs should only be needed
for entry points into the module, so the stubs should not need to be
modified often.

The remoteauth sources are still used for generating the API (as per
api_srcs in framework-connectivity-t), to avoid merge conflicts in the
API .txt files. But the implementation jar actually used on device or
shipped via module updates does not contain the API classes. In practice
this means that API stubs are generated through metalava from the
remoteauth sources, and these need to be built. So the API surface
itself cannot depend on V+ classes (APIs cannot take V+ classes as
arguments or return them). This is the case anyway, as
framework-connectivity-t is on the bootclasspath on T+; and in general,
module bootclasspath jars need to avoid depending on newer framework
classes as they may not exist on older devices using the module.

This change should only go to udc branches; note the merged-in below
refers to a change that was merged in all relevant branches, to ensure
that this change is not automerged anywhere.

Bug: 295788084
Test: m
Merged-In: I850242640f32bea3d46febcde5f3719619b1ea07
Change-Id: If533fef926ea0ad4e1e0b4d60c8f8e60f363fc1b