Refactor font strings to constants in QS

Cleaning up code by declaring and using font styles as constants rather
than directly as hard-coded strings. This is a follow-up CL to address
the comment in ag/30386360.

Flag: com.android.systemui.gsf_quick_settings
Bug: 379364381
Test: manual
Change-Id: Idf9fe8935c1182fe3e499aa19ac9540c419c4c39