Uploads: Remove an unnecessary static var from `wp_is_mobile()` to allow its...

Uploads: Remove an unnecessary static var from `wp_is_mobile()` to allow its direct and indirect use within unit tests. The static `$is_m
obile` var was only used to avoid a handful of calls to `strpos()`, which are exceptionally fast and result in no measurable increase in
processing time on each call to `wp_is_mobile()`.

See #35976, #20014

Built from https://develop.svn.wordpress.org/trunk@36813


git-svn-id: https://core.svn.wordpress.org/trunk@36780 1a063a9b-81f0-0310-95a4-ce76da25c4cd