[android] Convert splash_background JPEGs to WEBP

Summary:
Similarly to D4369, by doing this ahead of time we save some time during the build.

In this case we didn't do a lossless conversion, we did a lossy JPEG => WEBP conversion at 95/100 quality. This reduced file sizes by ~90% and saved ~3.2MB total.

Context: https://developer.android.com/studio/build/optimize-your-build#use_webp

Depends on D4369

Test Plan: Things continue to look as expected when launching app on Android emulator.

Reviewers: def-au1t, palys-swm, yayabosh

Reviewed By: palys-swm, yayabosh

Subscribers: ashoat, Adrian

Differential Revision: https://phab.comm.dev/D4370