The best (and easiest) way to add an email signature to Front on Android phones is to do it by using the desktop version of Front.
When you do that, your email signature will automatically be appended to all your emails sent out from your Front App on your Android. This is done by the server-side processing of your emails on the Front servers. Simply log in to your Front account using the desktop application for Mac or Windows, and install the email signature through there.
Once you complete the first step, your email signatures will be appended to all emails you send out from your phone using the Front App.