Add XML documentation for Android.Hardware.Input

The `Android.Hardware.Input` namespace contains **50** `To be added` placeholders in source XML documentation (excluding `docs\xml\index.xml`).

Example command:

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

## Update log

- 2026-09-10: Refreshed the placeholder scan from **48** to **50** (increased by **2**). Related documentation PRs: [#644](https://github.com/dotnet/android-api-docs/pull/644).

Start by running the provided rg scan against docs\xml, using the Android.Hardware.Input/** and ns-Android.Hardware.Input.xml paths while excluding docs\xml\index.xml. Review the 50 matching source XML documentation placeholders and replace them with accurate API descriptions. Done means the namespace scan no longer reports those placeholders and the documentation changes are complete.

Written by the indexing model from the issue text.