Remove android/apex_test.go use of buildDir

This test does not process any bp files it just needs a directory to
pass to TestConfig so it can use that to fake a PathContext. So, this
just replaces buildDir with a call to T.TempDir().

Bug: 182885307
Test: m nothing
Change-Id: I17c0435b7c45e4f8cb8b03a8ec8e57cfe397975b