Avoid race condition over ExportedSystemIncludeDirs

It is not clear whether this is causing the bug but it is a potential
source of problems and if it does not fix the bug then at least it will
have eliminated this as the culprit.

Bug: 157656545
Test: m nothing
Change-Id: I3abcb9ae501977924d0d514f4d9007ead24fe0a5