Load screenpool at the same time as we connect to screenchanged signals

Summary:
Otherwise we have a gap during load (waiting querying kactivities))
between screen pool being created and us connecting to the screen
changed signals, which in turn are used to update screen pool.

In particular the primary screen can get out of sync between the current
state and the screen pool.

Test Plan: Based on Christopher Feck's research and initial patch

Reviewers: Plasma

Subscribers: mart, rwooninck, fvogt, cfeck, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D3319

CCBUG:372099
CCBUG:371858
CBUG:371991
CCBUG:371819
CCBUG:371734