Don't build android.test.mock.stubs for unbundled builds

Unbundled builds use the prebuilt libs in /prebuilts/sdk/current and
system_current.

Bug: 73829976
Test: m -j ANDROID_BUILDSPEC=vendor/google/build/app_build_spec.mk

Merged-In: I61fe0d671778595f49be82d7c11caffc411f58a3
Change-Id: I61fe0d671778595f49be82d7c11caffc411f58a3
(cherry picked from commit ea93a9eb)