init: Make 'write_file' return bool to match 'read_file'.

The mismatch of return values makes reasoning about the correctness of
CLs like https://android-review.googlesource.com/317923 quite hard.

Bug: 33941660
Test: Init builds, HiKey boots.
Change-Id: Ia4b8a9af420682997b154a594892740181980921