Fix api_fingerprint.txt generation

Use the generated txt files as input instead
of globbing for now non-existent files.

Also add the system server api txt to the hash.

Bug: 179807354
Test: verify out/soong/api_fingerprint.txt changes now
Test: verify the command in the ninja file looks ok:
      grep api_fingerprint out/soong/build.ninja
Change-Id: I7a49ca134eb93b22537a4f3054285eb15f8c4256