[flag_changer refactor] Fix usages to conform with new API

Fix clients (and clients of their clients) to conform with the new
flag_changer API, namely:

- No need to specify the full path to the command line anymore, just
  the file *name* is enough.
- No need to add "quotes" for values in flags for Android (the same way
  as it works for desktop platforms).

BUG=catapult:#3112

Review-Url: https://codereview.chromium.org/2698703004
Cr-Commit-Position: refs/heads/master@{#451621}