[Expressive Design][SettingsLib] Fix IntroPreference summary width in Expressive theme

Removing `layout_centerHorizontal` resolves the conflict and allows the LinearLayout to correctly match its parent's width, ensuring the summary text can span the container as intended.

Bug: 428949137
Test: atest, visual Agent App page & some other pages that use IntroPreference (eg. App > Wifi Usage)
Flag: EXEMPT style update
Change-Id: I2a4a494af29a3a304319105412f53d8f2a5faeeb