fonts: Build different fonts.xml if EXCLUDE_SERIF_FONTS is true

* Google introduced a commit to exclude serif fonts ( for saving some system space)
https://android.googlesource.com/platform/external/noto-fonts/+/c1fe8849cbec6ccc0d315e88936485bb53fe34a3



Now if the flag is used, then it will not build the fallback serif fonts but the fonts.xml still have the fallback serif font file mapping which results in below logspam

E/Typeface: Error mapping font file /system/fonts/NotoSerifEthiopic-Regular.otf
E/Typeface: Error mapping font file /system/fonts/NotoSerifEthiopic-Bold.otf
E/Typeface: Error mapping font file /system/fonts/NotoSerifCJK-Regular.ttc and so on......

So, add a check for EXCLUDE_SERIF_FONTS flag and copy respective fonts.xml

* Also, Fix compiling generic fonts

Co-authored-by: neobuddy89 <neobuddy89@gmail.com>

Signed-off-by: Akianonymus <anonymus.aki@gmail.com>
Change-Id: Icef8903c2f5b5bb928c1cea47dca8ef5714b154b