fix import paths when compiling snapshots

When compiling snapshots we should import
- snapshot dir (e.g <foo-iface>/aidl_api/foo-iface/1)
- "current" of imported interfaces
  (e.g <bar-iface>/aidl_api/bar-iface/current)

Previously, the snapshot of generated sources(e.g. hidl2aidl_test_gen)
caused the build error due to the wrong import path.

Bug: 189288369
Test: aidl_unitttests
Test: m hidl2aidl_test_gen-freeze-api
      m
Change-Id: I92f8e290b89c737f4b16fc064e763bd47cd1a9da
6 files changed