Improve Tasker Integration UI

We've recently added integration with Tasker thanks to [an open source contribution](https://github.com/Automattic/pocket-casts-android/pull/431/). 🎉 The UI does not fit with the rest of Pocket Casts though.

### Some possible improvements:

* [x] The screen could use some margins around the edge ([PR](https://github.com/Automattic/pocket-casts-android/pull/475))

* [x] It would be nice to use icons to help differentiate the different options. Ideally, the filter options would look very similar to how they look on the filters tab in the app ([PR](https://github.com/Automattic/pocket-casts-android/pull/509))

* [ ] The Dropdown component doesn't indicate whether it can be scrolled to display more items ([relevant discussion on StackOverflow](https://stackoverflow.com/questions/69782653/jetpack-compose-possible-to-set-dropdownmenu-height-to-display-next-item)). Would be good to find a better way to present the list of options.

* [ ] Upon first opening the screen, the user just sees a text box with an icon or two and an ok button. Maybe there is a way we could make the relevant options more obvious to the user? 🤔

* [ ] Generally speaking, let's try and update the styling consistent with the rest of Pocket Casts

- ~Some of the fields accept Strings when only a number is a valid input. It would be great to require the correct input at the UI level. That would also allow us to pass around fewer raw Strings.~ (these have to be strings for Tasker: [comment](https://github.com/Automattic/pocket-casts-android/issues/471#issuecomment-1302311280))

Please go ahead and suggest other improvements. These are just a few that immediately came to mind for me.

Start with the Tasker integration screen and its Dropdown component, then compare the remaining UI with the filters tab referenced in the issue. Review the linked Stack Overflow discussion for dropdown presentation options. Done means the remaining unchecked improvements have an agreed scope and the Tasker screen has clearer options, scrolling, and styling consistent with Pocket Casts.

Written by the indexing model from the issue text.