Doping Mn<sup>2+</sup> in a New Layered Halide Double Perovskite PPA<sub>4</sub>NaInCl<sub>8</sub> (PPA<sup>+</sup> = C<sub>6</sub>H<sub>5</sub>(CH<sub>2</sub>)<sub>3</sub>NH<sub>3</sub><sup>+</sup>): Dimensional Reduction Accelerating Mn<sup>2+</sup> Dissolution and Separation for Efficient Light Emission