SystemUI: Fix crash issue when slide brightnesslevel in status bar

Getting panel holder by panel_holder in dual sim mode causes a crash.

Get panel holder by msim_panel_holder in dual sim mode and by panel_holder
in single sim mode. Move brightness mirror to front of behind scrim to
avoid it being covered by behind scrim.

CRs-Fixed: 737868

Change-Id: Id3021f879a416525804de5f786a522f4f23dc6f8