ListSectionHeader: fix text elision
When the styling was changed to add a separator to the right of the text, the line that conditionally sets Layout.fillWidth to true stopped working, causing text to never elide. We can't just fix it since then the separator line won't be visible. Instead, manually limit the text width to the width of the container it lives in.
No visual changes for text that fits. For text that's too long, it now elides:
BUG: 464681 FIXED-IN: 5.103