Skip initializing unused system bar painter

SystemBarBackgroundPainter invokes getSystemUiContext, that may trigger
initializing a window context and load resources. The operation could
cost ~5ms due to attachWindowContextToDisplayContent.

Since flag drawSnapshotAspectRatioMatch is introduced, there is no
longer a case that the snapshot starting window need to draw bars
background. So its initialization can be skipped as well.

Bug: 297502610
Flag: EXEMPT reduce unnecessary invocation
Test: Task snapshot starting window is normal from hot launch app.

Change-Id: Id6fa10da245845d5ed5f78558ab2f4193d2c7133