Automatically register plugins for FlutterFragmentActivity

[sarbagyastha]

Follow up of #15979

Description

#15979 only implemented automatic plugin registration for FlutterActivity. This PR implements the same for FlutterFragmentActivity.

Related Issues

Fixes flutter/flutter#54386

Checklist

• I read the [contributor guide] and followed the process outlined there for submitting PRs.
• I signed the [CLA].
• I read and followed the [C++, Objective-C, Java style guides] for the engine.
• I read the [tree hygiene] wiki page, which explains my responsibilities.
• I updated/added relevant documentation.
• All existing and new tests are passing.
• I am willing to follow-up on review comments in a timely manner.

Breaking Change

Did any tests fail when you ran them? Please read [handling breaking changes].

• No, no existing tests failed, so this is not a breaking change.
• Yes, this is a breaking change. If not, delete the remainder of this section.

[fluttergithubbot]

It looks like this pull request may not have tests. Please make sure to add tests before merging. If you need an exemption to this rule, contact Hixie on the #hackers channel in Chat.

Reviewers: Read the Tree Hygiene page and make sure this patch meets those guidelines before LGTMing.

[xster]

Ah superb. Thanks @sarbagyastha for taking this on. This is excellent. Please also add a test for your PR. See https://github.com/flutter/engine/blob/master/shell/platform/android/test/io/flutter/embedding/android/FlutterActivityTest.java#L148 for an example.

[ened]

@sarbagyastha this is great work! Will you continue on this PR?

[sarbagyastha]

Already Merged: #20865