Measurement of Top Quark Polarization in Top-Antitop Events from Proton-Proton Collisions at <span class="aps-inline-formula"><math display="inline"><msqrt><mi>s</mi></msqrt><mo mathvariant="bold">=</mo><mn>7</mn><mtext>?</mtext><mtext>?</mtext><mi>TeV</mi></math></span> Using the ATLAS Detector