built in key finder for multisrc/aniwatch/kaido

[rebelonion]

Fixes #2568
Alternate to: #2711

Tested on both Kaido and AniWatch

Uses regex to find the keys in the obfuscated js file.

This is slightly slower than pulling the key straight from GitHub, but not nearly as bad as de-obfuscating the JavaScript (not noticeable in daily use).
Obvious benefit of having always up to date keys.

Checklist:

• Updated extVersionCode value in build.gradle for individual extensions
• Updated overrideVersionCode or baseVersionCode as needed for all multisrc extensions
• Referenced all related issues in the PR body (e.g. "Closes #xyz")
• Added the containsNsfw = true flag in build.gradle when appropriate
• Have not changed source names
• Have explicitly kept the id if a source's name or language were changed
• Have tested the modifications by compiling and running the extension through Android Studio