Allow ReactActivityDelegate subclass to overrride mainComponentName

[augustl]

Currently, the final field mMainComponentName is used. This field is
initialized in the constructor of ReactActivityDelegate, and the
ReactActivityDelegate itself is initialized in the constructor of
ReactActivity. This means that the only way you can pass a main
component name to ReactActivityDelegate, is when your ReactActivity
subclass is constructed. At this point in the lifecycle of an
activity, the getIntent() value that the activity was initialized by
returns null, making it impossible to set the mainComponentName
dynamically based on data passed to the activity via an intent.

The mMainComponentName final field is also only used in onCreate of
the delegate, so it's not actually needed by the ReactActivityDelegate
at construction time. So the above limitation is not fundamental, it's
just a side effect of the API design.

By allowing subclasses of ReactActivityDelegate to implement a
getMainComponentName method, the subclass then has full control of how
to initialize the value. So an implementation of getMainComponentName
could be:

public String getMainComponentName() {
    return getIntent().getStringExtra("reactMainComponentName");
}

This commit doesn't remove anything and only adds a new method, so it
should be fully backwards compatible.

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Help us understand your motivation by explaining why you decided to make this change.

Test Plan

Related PRs

Release Notes

[ANDROID] [ENHANCEMENT] [ReactActivityDelegate] - Added ability for subclasses to override component name

Changelog:

[Android] [Enhancement] - Added ability for subclasses of ReactActivityDelegate to override component name

[facebook-github-bot]

@augustl I tried to find reviewers for this pull request and wanted to ping them to take another look. However, based on the blame information for the files in this pull request I couldn't find any reviewers. This sometimes happens when the files in the pull request are new or don't exist on master anymore. Is this pull request still relevant? If yes could you please rebase? In case you know who has context on this code feel free to mention them in a comment (one person is fine). Thanks for reading and hope you will continue contributing to the project.

[facebook-github-bot]

@augustl I tried to find reviewers for this pull request and wanted to ping them to take another look. However, based on the blame information for the files in this pull request I couldn't find any reviewers. This sometimes happens when the files in the pull request are new or don't exist on master anymore. Is this pull request still relevant? If yes could you please rebase? In case you know who has context on this code feel free to mention them in a comment (one person is fine). Thanks for reading and hope you will continue contributing to the project.

[pull-bot]

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⚠️	📋 Changelog - This PR appears to be missing Changelog.

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[cpojer]

Works for me. Sorry for the long time until we got back to you about this PR.

[facebook-github-bot]

@cpojer is landing this pull request. If you are a Facebook employee, you can view this diff on Phabricator.

[react-native-bot]

This pull request was successfully merged by @augustl in a8449c1.

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