Fix missing ub__signin_margin dimen in :core that broke 3P authentication consumers

[partha-uber]

Summary

Reported by a 3P consumer of com.uber.sdk2:authentication:2.0.3:

Android resource linking failed
AAPT: error: resource dimen/ub__signin_margin not found.

Worked around locally by defining <dimen name="ub__signin_margin">48dp</dimen>, but should be fixed in the SDK so other 3P developers don't hit the same error.

Root cause

• authentication/src/main/res/values/styles.xml defines UberButton.Login, which references @dimen/ub__signin_margin.
• :authentication depends only on :core (the published com.uber.sdk2:core).
• core/src/main/res/values/dimens.xml mirrors most resources from the legacy :core-android, but ub__signin_margin was never copied over — it only existed in :core-android.
• 3P consumers pulling com.uber.sdk2:authentication get :core transitively, but not the missing dimen — hence the AAPT error.

The other resources the same style references (ub__standard_padding, ub__secondary_text_size, @color/uber_black, the uber_logotype_* drawables, uber_button_background_selector_white) were correctly mirrored into :core, which is why no other resource errors surfaced.

Fix

Add <dimen name="ub__signin_margin">48dp</dimen> to core/src/main/res/values/dimens.xml, matching the value and position used in :core-android. One-line change, restores the mirror.

Test plan

• ./gradlew :authentication:assembleRelease builds successfully (verified locally — AAPT now resolves ub__signin_margin for the AAR a 3P consumer pulls).
• 3P repro: consume com.uber.sdk2:authentication (built from this branch) in a sample app and confirm Android resource linking failed no longer occurs.