Fix race condition when running tests

Tests running in parallel can trigger the race detector on
addLsdumpPath vs. reading the global paths in cc/makevars.go.
Move the lsdumpPaths variable into the Context instead of a
global variable.

Test: go test -race ./apex
Flag: EXEMPT bugfix
Change-Id: I0247358a5b3955d8e0e7d2ef54ce3942d973e948