This is fixed now via https://github.com/discourse/discourse/commit/886f4b589e87e55d8b364c503d495ac62f235a3b.
As Don noted, this separate Android-only issue is working as designed, we would like to have the same uniform experience between iOS and Android, and we believe that the composer should take up all the available space when the software keyboard is visible because keyboard plus composer already takes up a lot of space, there is no point in leaving, say, a 50px tall view of the post stream.
If you want to scroll the page and keep the composer half open, all you have to do is dismiss the keyboard using the specific button for that on Android (which is the downward-pointing chevron at the bottom-right corner in your screenshot).