system/gd: Inline filegroup declarations

Many filegroup declarations are used only once;
keeping them around prevents easy lookup of source
files in Android.bp (e.g. to find which target
builds a specific source file).

Bug: 331817295
Test: TreeHugger
Flag: EXEMPT, build change
Change-Id: I363676a8b31767298862fe13866403a38de97f0b