Polish humanize strings for Caption preferences page

- Add intro for usage
- Update footer description on limitations
- Update the default caption size to medium
- Update the caption size and style dynamically

Bug: 218409087
Test: make RunSettingsRoboTests
ROBOTEST_FILTER=CaptionAppearancePreferenceControllerTest

Change-Id: I6526250894cc6a3fd02e3d8c675280855b307a68