new font sizing strategy

Mostly go back to the previous approach of setting a default
font-size style that only affects unstyled text. On phone,
shrinking everything by 13% worked poorly for mail that was
pre-shrunk.

The slightly modified new approach more closely aligns with
gmail.com's behavior on desktop. Shrink unstyled text to
80%, and then upsize all text slightly such that regular text
ends up as 14px on phone and 16px on tablet.

Bug: 6684890
Change-Id: I7ae15cd4bf8d8f2cb0d9137c558bbfe13d0d6570
4 files changed