android: stabilize custom top bar button layout

[yedidyak]

Summary

• Stabilizes Android custom top-bar React button layout by keeping the bounded content-width discovery pass from android: fix custom top bar button measurement #8320, then remeasuring the button with an exact final content width.
• Preserves title-width behavior from android: fix custom top bar button measurement #8320 so custom buttons reserve their measured content width instead of a forced actionBarSize width.
• Updates regression coverage for missing dimensions, explicit dimensions, RTL, and zero-width parent fallback behavior.

Validation

• git diff --check
• ./gradlew :react-native-navigation:testDebugUnitTest --tests com.reactnativenavigation.views.TitleBarReactButtonViewTest --tests com.reactnativenavigation.views.TitleAndButtonsContainerTest — SUCCESS (37 successes, 0 failures, 0 skipped)

Not Run

• Full Android unit/instrumentation suite; this change is scoped to the top-bar button measurement path and the focused affected tests passed.

Risks

• Low to medium: scoped to Android custom React top-bar buttons without explicit width.
• The main behavior to watch is custom button width measurement for components whose content changes after initial layout.

Review Context

• This is a follow-up to android: fix custom top bar button measurement #8320. The first pass still uses bounded AT_MOST width to prevent title truncation.
• The second pass restores an exact final RN/Yoga layout box to address vertical alignment regressions in custom buttons.
• Pre-PR review found no cleanup items, blockers, or creator-intent questions.

Suggested Reviewers

• Yedidya Kennard — latest relevant change owner for TitleBarReactButtonView and the new regression tests in android: fix custom top bar button measurement #8320.