Make UsersPager::requestedGroup public
This was made protected in I66ed64627dc475dc85da9d858a19317435d05532,
but some extensions are using it from outside (see e.g. failure at I850d1f4aa2bcba9cf0375e290a511c8337ee9499).
Change-Id: I8482e71d4d5e3763d41437a26e1b0fe767c0124c
(cherry picked from commit 2d33abad9280f338e625768a408b067763a3064e)