Allow installing fake vendor packages

This line in base_rules.mk requires a variable that's qualified by
target/host, the partition, and the module class. There didn't exist
one for target/vendor/fake, so fake vendor modules failed to compile.
Add a TARGET_OUT_FAKE so we can use fake vendor modules. This is aimed
at using install_symlink on the vendor partition.

https://cs.android.com/android/platform/superproject/main/+/main:build/make/core/base_rules.mk;l=279;drc=c67e769b8c44eeb9bb851f20003b63d35193e155

Bug: 205632228
Test: presubmits
Change-Id: If4618f90cebbafcd44e88e38513033282c37c1b7