Fix handling of "android.test.mock" in class loader context.

Mimick the way PackageManager handles it at runtime: do add it to class
loader context for apps with targetSdkVersion < 30, but only if
"android.test.runner" is used. Previously it was not added at all.

Test: lunch aosp_cf_x86_phone-userdebug && m
Bug: 132357300
Change-Id: I4c06635277ab13e21069b9fa0b46eb6a2547dfdd