Use PwriteFully and write image header last

Prevent corrupted images if dex2oat gets killed or if the image
writer is interrupted during writing.

Bug: 22858531
Bug: 27561308

(cherry picked from commit dba5a70977da0a28cec2bfc8261d52a177738477)

Change-Id: If4f2c43bcc3cf918b5d2780f1709225b5a4ce116
4 files changed