Ensures the slop value is fixed cross lines.

In Q, the slop is a fixed value calculated based on
TextView.getLineHeight().
In R+ (before this cl), the slop is a dynamic value based on the height
of the current line.

It's been confirmed as the root cause of the regression bug b/159528374.

This cl changes the slop value back to the fixed value.

Bug: 159528374
Test: Verified locally.
Change-Id: I182d22779f1c1155ae2e0b0deb0455bd270b2fd9