is_plugin_active() returns based on current site in Network Admin

[kawauso]

is_plugin_active() uses the 'active_plugins' option, which is site-specific, as well as is_plugin_active_for_network().

Most obviously affects the Plugins section in Network Admin, which displays both the current site's plugins as well as those network activated as being active.

Use is_network_admin() check before using 'active_plugins' option

Spread code over multiple lines for readability

We wouldn't put this logic into is_plugin_active. Where I would put the logic, already has the logic -- It looks like we've properly accounted for is_network_admin() in WP_Plugins_Table::display_rows(). I also can't reproduce this.

(In [15870]) Add some more network admin logic to the plugins list table. fixes #15166.