Redraw default background

Make images crisper, fix aspect ratio. PNGs created using this script:

for x in ldpi-120 mdpi-160 hdpi-240 xhdpi-320 xxhdpi-480 xxxhdpi-640; do
    day=$(mktemp --suffix=.svg)
    sed 's/#000000/#dde1e6/g' artwork/app_background.svg > "$day"
    inkscape -b '#e4e9ee' -o app/src/main/res/drawable-${x%-*}/app_background.png -w ${x#*-} "$day"
    rm "$day"

    night=$(mktemp --suffix=.svg)
    sed 's/#000000/#181b1e/g' artwork/app_background.svg > "$night"
    inkscape -b '#1c2024' -o app/src/main/res/drawable-night-${x%-*}/app_background.png -w ${x#*-} "$night"
    rm "$night"
done

Densities match Table 1 here:
https://developer.android.com/training/multiscreen/screendensities#TaskProvideAltBmp

Signed-off-by: Andrew Nayenko <relan@users.noreply.github.com>

Author: relan