[res] Add named padding to ResTable_config

Follow the way the structs are defined in this file and
explicitly pad the structure with a named field.

+ add some static_assert statements to make sure nothing
  accidentally breaks binary structure of configs.

Test: build + boot
Flag: EXEMPT minor refactoring
Change-Id: I7e93aa0e92b080d0dcb77c790e5341f2342d7d2d