Inline UCO activation

Depends on: #11094 (WithinNewObjectScope guard)

Scope

• remove TrimmableTypeMap.ActivateInstance() from the runtime path
• inline activation directly in generated nctor_*_uco wrappers
• keep the generated, non-reflective activation flow for both leaf and inherited activation ctors
• keep the small PEAssemblyBuilder.EmitBody(..., useBranches: true) plumbing needed by the emitted IL
• keep the no-op open-generic UCO registration behavior needed for valid generated wrappers

Not in this PR

• the WithinNewObjectScope guard itself — that lives in [TrimmableTypeMap] Fix UCO constructor activations #11094
• reflection fallback behavior — removed in [TrimmableTypeMap] Fix UCO constructor activations #11094 and not reintroduced here
• unrelated trimmable typemap follow-up work

Generated IL shape

[UnmanagedCallersOnly]
static void nctor_0_uco(IntPtr jnienv, IntPtr self)
{
    if (JniEnvironment.WithinNewObjectScope)
        return;

    var obj = (MyActivity)RuntimeHelpers.GetUninitializedObject(typeof(MyActivity));
    obj.BaseCtor(self, JniHandleOwnership.DoNotTransfer);
}