Read font attributes from buffer.

minikin::Font::typeface() is expensive because it will open the font file.
Font attribute getters should avoid calling it.

Bug: 188201287
Test: atest UpdatableSystemFontTest
Test: atest CtsGraphicsTestCases:FontTest
Test: atest CtsGraphicsTestCases:SystemFontsTest
Change-Id: Ic8554f6dfacbe27ddfea6b375633c96bced2cc09