refactor(android): reuse one flag for top-priority exclusive spans

[jnowakow]

What/Why

Follow-up to #938, addressing its review comments (1, 2).

#938 set HeadingSpan with Spanned.SPAN_EXCLUSIVE_EXCLUSIVE or Spanned.SPAN_PRIORITY, the same value as SPAN_FLAGS_CONTAINER_BACKGROUND. This PR renames that constant to the more generic SPAN_FLAGS_EXCLUSIVE_EXCLUSIVE_PRIORITY and uses it in both places.

• SpanFlags.kt: rename and generalize the doc comment. It now covers both uses: a block span that must not override its inline spans (heading) and a container background that must sit under inline pills (blockquote).
• HeadingRenderer.kt: use the constant.
• BlockquoteRenderer.kt: use the renamed constant. The blockquote-specific reason now lives in a comment at the call site.

The changes go into both react-native-enriched-markdown and the standalone enriched-markdown-android. The flag values don't change, so rendering doesn't either.

Testing

• ktlintCheck and compileDebugKotlin pass for :react-native-enriched-markdown and the standalone :ui module.

PR Checklist

• Code compiles and runs on iOS: not applicable, Android-only change.
• Code compiles and runs on Android
• Updated documentation/README if applicable: not applicable, internal refactor.
• Ran example app to verify changes: not run, flag values are unchanged.
• E2E tests are passing: not run.
• Required E2E tests have been added (if applicable): not applicable.