fix(HCT): Scale the rect corner radius with the font size

This better aligns with our UXD mocks, and should be more
noticeably round to users.

Test: See screenshots in the bug
Test: atest LayoutTest
Test: atest TextViewHighContrastTextTests
Fix: 401026064
Flag: com.android.graphics.hwui.flags.high_contrast_text_small_text_rect
Change-Id: I4bf6da3a44721980fe9a1ba06fe6c6f17d8cdd4c