enable exportable check

If a library is built to mode exported, the flag declaration has to be
set with exportable true. The reason for this requirement is only flags
in the exportabel declarations will be repackaged. To prevent forgetting
adding this attribute, this exportable attribute is required to be set
for all exportable declarations.

Test: m
Bug: 310504781
Change-Id: I3ddafe0b212dabe86aa5000b1b76c7c412acc93c