Removing MainActivity from plugin

[fidelisakilan]

Having MainActivity class within plugin launching app through share causes issue when app have native code added to kotlin class.

Thanks to @HaloWang for pointing out the issue

Related issues:
#9 (comment)
#9 (comment)

[fidelisakilan]

One limitation will be, we have to add intent filter manually in the parent app

            <intent-filter>
                <action android:name="android.intent.action.VIEW" />
                <action android:name="android.intent.action.SEND" />
                <category android:name="android.intent.category.DEFAULT" />
                <data android:mimeType="image/*" />
            </intent-filter>
            <intent-filter>
                <action android:name="android.intent.action.VIEW" />
                <action android:name="android.intent.action.SEND" />
                <category android:name="android.intent.category.DEFAULT" />
                <data android:mimeType="video/*" />
            </intent-filter>

[bhagat-techind]

@fidelisakilan Thanks for contributing, I have merged your PR with minor update. So I closing it now.