[Settingslib] Fix summary text centering issue in IntroPreference

Summary text in IntroPreference is implemented using CollapsableTextView. For some reason, in the context of GMS Core settings screen, the existing text centering code in CollapsableTextView (view.gravity = Gravity.CENTER) did not work. This CL adds view.textAlignment = View.TEXT_ALIGNMENT_CENTER, which was observed to solve the centering issue on a real device.

Bug: 378494253
Test: manual
Flag: Flag: EXEMPT bug fix
Change-Id: Ib49b3da4240c1bc84eb1684f80660c6530e7c9b7