Pixiv: fix search repeats itself

[lamaxama]

Close #4923

PixivFilters is a delegation that acts as a proxy to MutableList. However, the hashcode is not proxied to MutableList. ref: https://youtrack.jetbrains.com/issue/KT-15445

Therefore, the hashcode of filters here is not the hashcode of the list, even if two filters have the same content.

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 isNsfw = 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
• Have removed web_hi_res_512.png when adding a new extension