Rename the disable_shade_expands_on_trackpad_two_finger_swipe flag

This flag was originally submitted without the bug fix metadata, and
the only way to fix the problem is to rename it.

Test: building and running automated tests is sufficient
Bug: 371224114
Flag: com.android.systemui.disable_shade_trackpad_two_finger_swipe
Change-Id: Icfaa279167432fe283b70d058d9fcc7accac9ff4