[SB] Ensure StatusBarBoundsProvider gets started even with root_mod on.

Previously, CollapsedStatusBarFragment was responsible for starting all
the HomeStatusBarComponent.Startable classes, of which there was only 1:
StatusBarBoundsProvider. But that fragment is deprecated when
status_bar_root_modernization is on, so we need to start and stop that
class another way.

Fixes: 417965212
Flag: com.android.systemui.status_bar_root_modernization
Test: atest StatusBarModeRepositoryImplTest
Test: manual: In a letterboxed app, add & remove icons in the status bar
in order to make the icons overlap or not overlap the app. Verify the
status bar adds a scrim when the icons overlap the app and removes the
scrim otherwise (see video in bug). Verify not letterboxed apps aren't
affected

Change-Id: I2fe234ef01dd815bb19f4dc8232b59b5d2e53af3