Make strike-through respect kerning at boundaries

Also clean up lower-level drawing of strike-through in Cavas.cpp.

We still cannot use the strike-through information from the font
since Skia doesn't provide it, so we are going with the default
values.

Test: Manual
Test: bit CtsTextTestCases:*
Test: adb shell am instrument -w -e package android.text com.android.frameworks.coretests/android.support.test.runner.AndroidJUnitRunner
Bug: 32907446
Change-Id: Iee6f8102a35eba0ff4127dbbef189529ab573e6d