Remove jcenter references from gradle build

[iAmWillShepherd]

@Davete0302 Yes as a work around for now you should be able to replace jcenter() with mavenCentral() in all your build.gradle files. Almost all libraries hosted on jcenter are also on Maven Central. OneSignal will be making an update to remove all jcenter() references soon as well.

Originally posted by @jkasten2 in #516 (comment)

[iAmWillShepherd]

@jkasten2 I created this issue to track the work you alluded to in #516. People are still running into this issue, with the last report occurring a little over a week ago. If the fix is low effort, I may be able to do it, but I'd be lying if I claimed to have any familiarity with the Java ecosystem of tools.

[achalkias]

Replacing jcenter() with mavenCentral() does not fix this
still getting the same error

Could not GET 'https://google.bintray.com/exoplayer/androidx/cardview/cardview/maven-metadata.xml'. Received status code 502 from server: Bad Gateway
Disable Gradle 'offline mode' and sync project

[jkasten2]

This is now fixed and is part of the OneSignal-Flutter-SDK 3.2.8 Release.