Make outputApexFile relative to module dir in mixed builds

Otherwise when other modules use the apex as data (for example, in
tests) they get the full path from the root of the workspace to the
apex.

Fixes: 276416485
Test: atest timezone_data_e2e_tests:com.android.tests.apex.TimezoneDataHostTest#testStageActivateUninstallApexPackage -- --abi x86_64 then tree out/host/linux-x86/testcases/timezone_data_e2e_tests
Change-Id: Id740856794770df2edc55dfff5f6f938fc31d0c5