android: fix custom top bar button measurement

[yedidyak]

Summary

• Fix Android custom top bar React button fallback measurement to use bounded AT_MOST specs instead of forcing unspecified dimensions to actionBarSize.
• Preserve explicit width / height behavior as exact dp sizing.
• Add Robolectric coverage for intrinsic sizing, explicit sizing, bounded fallbacks, RTL bounded specs, and title width regression behavior.

Validation

• git diff --check — passed.
• Code review pass — clean; no sensitive files or unrelated changes found.

Not Run

• cd playground/android && ./gradlew :react-native-navigation:testDebugUnitTest --tests '*TitleBarReactButtonView*' --tests '*TitleAndButtonsContainerTest*' — blocked because node_modules/@react-native/gradle-plugin is missing in this checkout.
• yarn install --immutable — attempted to restore dependencies, but Yarn failed during fetch with repeated RequestError EBADF, so local Android unit tests could not be started.

Risks

• Low to medium: scoped to Android top bar custom React button measurement.
• Main compatibility risk is custom button components that unintentionally relied on the temporary default 48dp exact sizing when no dimensions were provided.

Review Context

• This keeps the RTL fix's bounded-measurement property by avoiding UNSPECIFIED, but restores intrinsic-width behavior so buttons do not unnecessarily shrink the title area.
• Height still falls back to actionBarSize when the parent provides no height; width falls back to screen width only as a non-zero bound.

Suggested Reviewers

• @markdevocht — latest and dominant editor on the Android top bar button/title measurement code touched here.