Promoting file_exists to AndroidDevice class method. am: b51b6d04a3 am: 8af563403e am: c512bf0896

Original change: https://android-review.googlesource.com/c/platform/tools/test/connectivity/+/1573075

MUST ONLY BE SUBMITTED BY AUTOMERGER

Change-Id: Idb079b45d06bffd9d7357ec9c7ddbfa403718d80
tree: 9713287a84066e4f6dac1e2b448512aef0f2d461
  1. acts/
  2. acts_tests/
  3. tools/
  4. wts-acts/
  5. Android.mk
  6. OWNERS
  7. PREUPLOAD.cfg