Keyboard shortcuts: icons for application group

Query the package manager and if the user has default
applications for categories (email, browser, etc.) then
show the default app icon else do not show an entry in
the group.

PS: tested with 10x apps and did not notice any visible
jank or delay.

Bug: 27453985
Change-Id: Ic8b73419518c81e3b19278341f83e91bb547c5f7