[TrimmableTypeMap] Add trimmable [Export] and [ExportField] callback support

Documentation/release-notes/11123-trimmable-typemap-export-attribute.md

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+#### App build and runtime
+
+- [GitHub PR #11123](https://github.com/dotnet/android/pull/11123):
+  Add trimming-friendly support for the `[Export]` and `[ExportField]`
+  attributes to the trimmable type map (`_AndroidTypeMapImplementation=trimmable`).
+
+  Customer-visible behavior notes when building with
+  `_AndroidTypeMapImplementation=trimmable`:
+
+  - `[Export]` and `[ExportField]` are supported on user types deriving from
+    _Java.Lang.Object_ (and other registered Java peers). The build emits
+    `[UnmanagedCallersOnly]` wrappers and registers them with the matching
+    Java _native_ methods directly — no dynamic codegen at runtime.
+
+  - The _Mono.Android.Export_ assembly is excluded from the trimmable build
+    because its `DynamicMethod`-based codegen is incompatible with trimming
+    and AOT. Applications that only consume the `[Export]` /
+    `[ExportField]` attributes are unaffected (the attribute types are
+    defined on the user assembly's referenced types).
+
+    Apps or libraries that directly call APIs in the _Mono.Android.Export_
+    assembly are not supported under the trimmable type map.
+
+  - User-visible managed constructors are now invoked when Java instantiates
+    a peer via a registered Java constructor (for example, the `(Throwable cause)`
+    constructor on `Throwable` subclasses and parameterized `[Export]` constructors).
+    Previously the trimmable path always built the peer via the activation
+    constructor `(IntPtr, JniHandleOwnership)`, which skipped any user
+    constructor body.
+
+  - Unhandled managed exceptions thrown from an `[Export]` method are routed
+    through `JniRuntime.OnUserUnhandledException` (matching the modern
+    _JavaInterop_ exception contract). The original managed exception type
+    and message are preserved across the JNI boundary. This differs from
+    the legacy _LLVM IR_ type map path, which converted the exception into
+    a _Java.Lang.Throwable_ via `AndroidEnvironment.UnhandledException`.