SearchActivity tab names are hardcoded

**Summary:** 

`SearchActivity` has the tab names ("MEDIA" and "CATEGORIES") hardcoded. These should be extracted into `strings.xml` with appropriate names so they can be translated.

See lines [96 and 98](https://github.com/commons-app/apps-android-commons/blob/e36a71a584110dc4e325929a0daee94857e10b96/app/src/main/java/fr/free/nrw/commons/explore/SearchActivity.java#L96).