Only use a fixed timestamp when packaging.

[1] changed to use a fixed timestamp for all the generated images
(either the one under $ANDROID_PRODUCT_OUT/ or the one added into the
target_files zip). It makes 'adb sync' after 'fastboot flashall' to
resync all the files. This CL changes to use a fixed timestamp only when
packaging into the target_files zip.

[1]: commit 052ae354

Bug: 24377993
Bug: 24536411
Change-Id: I209318966109fd4902468dfe96caf777464f9fe1