Investigate conversion of hard-to-replace PNGs to SVG

Almost all graphics in the app are SVG instead of PNG now. There are just a few left that are hard to convert.
It may be that some or all of these are *impossible* to convert, that is unknown and part of the task.

It is certain that this is not a high priority though, as the number of remaining PNGs is so small that any additional compiled app size benefits will be very small.

`ankidroid_txt` (I have svg format of anki txt if someone wants to try )
`drawer_shadow.9`
`widget_bg_number.9.png`
`widget_bg_small_finish.png`
`widget_bg_small.png`
`widget_bg_time.9.png`

_Originally posted by @vinishhub in https://github.com/ankidroid/Anki-Android/issues/8707#issuecomment-828659145_

@vinishhub - please attach your SVG of ankidroid_txt here if you could? Thank you!