change base url in js-api

Purpose / Description

The post base url changed, so it needs to modify for js api also.

ankidroid.postBaseUrl added to js base url.

Tested on device with this deck, AnkiDroid JS API Test Deck

Please, go through these checks before submitting the PR.

[x] You have a descriptive commit message with a short title (first line, max 50 chars).
[x] You have commented your code, particularly in hard-to-understand areas
[x] You have performed a self-review of your own code
[ ] UI changes: include screenshots of all affected screens (in particular showing any new or changed strings)
[ ] UI Changes: You have tested your change using the Google Accessibility Scanner