Flutter: fix unknown android license status issue

Because of android SDK not correctly working with java 11, when run flutter doctor we’ll get the following error:

$ flutter doctor
...
Android license status unknown.
Try re-installing or updating your Android SDK Manager.
...

To fix this, just follow the instructions to download the older version of Java.

If after switching java version flutter doctor still complains about licenses not accepted, just run the following:

yes | sdkmanager --licenses

It will automatically accept all the available licenses (of course you should read those before accept them).