Handwriting gesture line margin

Point-based handwriting gestures are supported for points within text
line bounds. With this change, they will also be supported for points
which are outside the line bounds but within a margin of the line bounds.

The margin is added to ViewConfiguration so that the value can be
accessed by other UI toolkits so that they can implement the same
behavior.

The dimension resource is added as a @SystemApi so that OEMs may
override the value.

Bug: 243967209
Test: atest android.widget.cts.TextViewHandwritingGestureTest
Change-Id: I0e64f20c670808580346a485d06a0da0f21d3ed2