Document that android.presplash_lottie overrides presplash.filename

[Julian-O]

On every platform, you can specify a presplash-filename. The documentation explains the limited formats supported; JPG and PNG.

However, it doesn't mention there that on Android, you can specify android.presplash_lottie to support a third format, and that if you specify both, the lottie version wins.

Similarly, the default.spec doesn't mention the relationship between the two.

Clarify that relationship in each point of the documentation.