Add fallback attribute for custom fallback in fonts_customization.xml

Bug: 430485331
Test: atest SystemFontsUniqueNameTest FontManagerTest FontFamilyUpdateRequestTest
Test: atest VariableFamilyTest UpdatableSystemFontTest TypefaceSystemFallbackTest
Flag: com.android.text.flags.custom_fallback_for_customization
Change-Id: Ia4e217fe7a13318d9c06e43c2162685e17267746