The meta description isn’t part of the Twenty Fifteen theme.
You mentioned the Yoast plugin in a previous question yesterday, so I guess the meta description comes from there.
According to their API page, there’s a filter called wpseo_metadesc
that might be what you’re looking for.
Here’s an untested example:
/**
* Change the Yoast meta description for Android devices on the front/home page.
*/
add_filter( 'wpseo_metadesc', function( $description )
{
return
( is_front_page() || is_home() )
&& isset( $_SERVER['HTTP_USER_AGENT'] )
&& false !== stripos( $_SERVER['HTTP_USER_AGENT'], 'android' )
? 'Some Android description!'
: $description;
}, PHP_INT_MAX );