title: Add Jetpack-Compose to projects uses API below 21
body: <p>I know that minimum API level 21 is a must for enabling Jetpack Compose in a project.</p>
  <p>I want to use JetPack Compose in my existing project. It uses <code>minSdkVersion=17</code> and I can't add compose dependencies to <code>build.gradle</code>.</p>
  <pre><code>uses-sdk:minSdkVersion 17 cannot be smaller than version 21 declared in library [androidx.activity:activity-compose:1.3.0-alpha03]
  </code></pre>
  <p>My goal is to use Compose for users that have Android API &gt;= 21.</p>
  <h3>How I achieve this goal?</h3>