message: fix(android): use API 21 as minimum version so KeyguardManager is fully supported
diff: <allow-intent href="https://play.google.com/*" />
  <allow-intent href="https://airgap.it/*" />
  <preference name="ScrollEnabled" value="false" />
  - <preference name="android-minSdkVersion" value="19" />
  + <preference name="android-minSdkVersion" value="21" />
  <preference name="BackupWebStorage" value="none" />
  <preference name="SplashMaintainAspectRatio" value="true" />
  <preference name="FadeSplashScreenDuration" value="300" />
  