Start handwriting for stylus tap at end of line

To make it easier to insert a period, we will initiate handwriting when
there is a stylus tap at the end of a line (if the text field is already
focused and not empty with the cursor at the end of that line).

Bug: 323423905
Test: atest StylusHandwritingTest
Change-Id: Ide9300b36ae1b8a98faa2283d1c7539c44375e2d