fix(android): Use cdvPluginPostBuildExtras instead of ext.postBuildExtras (CB-14163)

[merrygobyebye]

Use cdvPluginPostBuildExtras instead of ext.postBuildExtras

Platforms affected

Android

What does this PR do?

1. Adds a reference to the build-extras.gradle file in plugin.xml so that the code in build-extras.gradle will run with the main Cordova build.gradle.
2. Replaces incorrect usage of ext.postBuildExtras with usage of cdvPluginPostBuildExtras.

What testing has been done on this change?

Added the modified cordova-plugin-file to a Cordova project and verified that the code in build-extras.gradle ran during the execution of the command cordova build android.

Checklist

• Reported an issue in the JIRA database
• Commit message follows the format: "CB-3232: (android) Fix bug with resolving file paths", where CB-xxxx is the JIRA ID & "android" is the platform affected.
• Added automated test coverage as appropriate for this change.

[janpio]

Run current tests

[janpio]

CI: Once more please.

[BrianMulhall]

is this change needed? I am curious if I should be running the code
defined in the build-extras.gradle file

[breautek]

For context, the original issue this PR addresses is on jira at https://issues.apache.org/jira/browse/CB-14163

The motivation appears to be a bug if multiple plugins are trying to use ext.postBuildExtras where one will overwrite the other (as claimed in the bug report).

[BrianMulhall]

if the plugin.xml doesnt have the change in this pull request the build-extras.gradle wont exist in the build output. So I am really wondering if the contents of the build-extras.gradle needs to be run at all. Cause if it does then i think this change is important