init: Fix leaking fd and error print

read_file() does not close its fd if either stat() fails or the file
has group/world writable permissions.

Use unique_fd to ensure that all return paths close the fd and make
the same change to write_file() for consistency.

Replace PLOG() with LOG() after a simple if conditional, that does not
set errno.

Old:
init: skipping insecure file '/data/bootchart/header': No such device or address
New:
init: skipping insecure file '/data/bootchart/header'

Test: Cause an invalid file read and check the error log
Test: Ensure non-error read_file() and write_file() work
Change-Id: Ib15d94e38362e335d671d30b36aa5605254ec7ab