Touchpad: add final strings for tap dragging

The title string was approved by UX as it is, so mark it as ready to
translate, and add a summary line.

Screenshot: go/b-324058706-strings-screenshot

Bug: 324058706
Bug: 321978150
Change-Id: Iaa757be16b58b23fba8a6841621ad8000b51c250
Test: Open touchpad settings, check new summary line appears