Abolish AutoCloseFD.h in favour of unique_fd

Android has a standard way to do what AutoCloseFD.h does, so use that
instead. Refactor before work on the bug.

Bug: 36029169
Test: Deleted a user and checked that secdiscard logs looked good.
Change-Id: I5d8bedfb3fa1f032fd2bced88b1b561e4a8c2ff4
3 files changed