Clipboard Plugin: Added support to send clipboard to multiple devices on Android 10 and later.

Summary

Workaround for clipboard plugin on Android 10 https://invent.kde.org/kde/kdeconnect-android/-/merge_requests/127 only worked when exactly one device was connected. This patch adds support to send clipboard to all connected devices as was done on previous Android versions.

I have also added the needed documentation, comments for ClipboardFloatingActivity and refactored some variables to better names too.

Test Plan

Before:

The action button to send the clipboard in the foreground notification was not added when there was more than one device was connected.

After:

The action button now supports sending clipboard to multiple devices.