Add XML documentation for Android.Icu.Number

The `Android.Icu.Number` namespace contains **629** `To be added` placeholders in source XML documentation (excluding `docs\xml\index.xml`).

Example command:

```powershell
rg -n -i --glob 'Android.Icu.Number/**' --glob 'ns-Android.Icu.Number.xml' 'to be added' docs\xml
```

## Update log

- 2026-09-10: Refreshed the placeholder scan from **625** to **629** (increased by **4**). Related documentation PRs: [#644](https://github.com/dotnet/android-api-docs/pull/644).

Start by running the provided rg command against docs\xml to enumerate the 629 placeholders under Android.Icu.Number/** and ns-Android.Icu.Number.xml. Read the surrounding XML and corresponding Android.Icu.Number API declarations to determine each member's meaning. Done means replacing the namespace's "To be added" entries with accurate documentation and confirming the scan returns no remaining placeholders.

Written by the indexing model from the issue text.