Keep AVD info in local goldfish instance directory

- Have emulator command create AVD info files in instance directory by
  setting ANDROID_TMP.
- Add creation_timestamp.txt to local goldfish instance directory.
  `acloud delete` deletes the timestamp file only. The AVD info and log
  files are kept until the next `acloud create`.

Bug: 140786679
Test: acloud-dev list --verbose
Change-Id: I95c06adacd1ecc637b33988bb3f3bde94b128cef
6 files changed