Add XML documentation for Android.Media.Session

The `Android.Media.Session` namespace contains **278** `To be added` placeholders in source XML documentation (excluding `docs\xml\index.xml`).

Example command:

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

## Update log

- 2026-09-10: Refreshed the placeholder scan from **274** to **278** (increased by **4**). Related documentation PRs: [#613](https://github.com/dotnet/android-api-docs/pull/613), [#644](https://github.com/dotnet/android-api-docs/pull/644).

Run the provided ripgrep command against docs\xml to enumerate the 278 placeholders under Android.Media.Session, excluding docs\xml\index.xml. Read the surrounding XML documentation entries to determine the missing API descriptions. Done means the namespace and its related files no longer contain the identified “To be added” placeholders.

Written by the indexing model from the issue text.