[android_alarm_manager] remove MainActivity references

[cyanglaz]

Description

Following up 899744f
The MainActivity was removed but there were still references left.

Related Issues

fix ci.

[GaryQian]

LGTM! Thanks for catching this!

[cyanglaz]

Landing to fix CI.

[vkammerer]

@cyanglaz @GaryQian : is there still additional cleanup to be done in this plugin following 899744f ?

It looks like we could:

• set the application name to the default FlutterApplication in AndroidManifest.xml

<application android:name="io.flutter.app.FlutterApplication" >

• delete Application.java
• delete EmbeddingV1Activity.java
• Replace the whole section at https://github.com/flutter/plugins/blob/master/packages/android_alarm_manager/README.md#using-other-plugins-in-alarm-callbacks with a section explaining that this plugin only supports Flutter >= 1.12.13+hotfix.5

This would simplify a lot the usage of this plugin, which can still be quite confusing for new users.

If you think these tasks can be done I'd be happy to open a PR.
I may be missing other cleanup tasks that would also need to be done, if so please let me know.

[vkammerer]

I shouldn't have posted this question in this thread. Moving my question to #2613 (comment)