Skip buffer equality check for SystemFonts

All system font files are file backed and stored in the unique place,
so no need to compare the buffers during creating sets of system fonts.

Bug: 188201287
Test: atest FontFamilyUpdateRequestTest
Test: atest FontListParserTest
Test: atest FontManagerTest
Test: atest NativeSystemFontTest
Test: atest PersistentSystemFontConfigTest
Test: atest SystemFontsTest
Test: atest SystemFontsUniqueNameTest
Test: atest UpdatableFontDirTest
Test: atest UpdatableSystemFontTest
Change-Id: I4a4680937988c013aaa4e4fa729e61ac8514c513