try harder to get the exec name for flatpaks

so gnome-shell's application search does not
show zero result for an application that is
installed both by distro package and by flatpak.

Fixes issue gnome-shell#1013
8 jobs for 1013-search-evince-or-totem-zero-result-from-flatpaks in 22 minutes and 41 seconds (queued for 3 seconds)
latest
Status Name Job ID Coverage
  Build
passed cross-android_api21_arm64 #234053

00:01:10

passed cross-android_api28_arm64 #234054

00:01:08

passed cross-mingw64 #234055

00:01:37

passed fedora-x86_64 #234052

00:04:30

passed freebsd-11-x86_64 #234058
freebsd-11

00:03:44

passed msys2-mingw32 #234056
win32

00:12:13

passed vs2017-x64 #234057
win32

00:08:29

 
  Coverage
passed coverage #234059

00:01:58

74.8%