Android Capacitor Support | Add Default for ANDROID_APP_KEY + ANDROID_APP_SIGNATURE

[Lindsor]

The Apptentive Cordova plugin does not currently allow setting ANDROID_APP_KEY and ANDROID_APP_SIGNATURE through the build.gradle files if we are using non-managed projects.

With Capacitor as a very popular alternative to Cordova providing the ability to set these two values in the build.gradle file should be supported.

[Lindsor]

Created PR: #118

This will allow us to set the values in the build.gradle file like so:

// app/build.gradle

android {
  // ...

  defaultConfig {
    // ...
    manifestPlaceholder {
      APPTENTIVE_ANDROID_APP_KEY: "<THE_ANDROID_APP_KEY>"
      APPTENTIVE_ANDROID_APP_SIGNATURE: "<ANDROID_APP_SIGNATURE>"
    }
  }
}