Reland "Keep timestamp in aar.py and copy_ex.py when file is not changed"

This is a reland of 53ed5722bc73a243e24880f5b5554da1d3017af6

But only reland the change for copy_ex.py
Reverting aar.py fixes the build failure in Android builder.

Original change's description:
> Keep timestamp in aar.py and copy_ex.py when file is not changed
>
> This removes unnecessary build steps in incremental build.
>
> Also this mitigates crbug.com/860251 a bit.
>
> Bug: 810298, 860251
> Change-Id: Ia6aedfea692611a08b1f9a6a2c3963bb69cf32bc
> Reviewed-on: https://chromium-review.googlesource.com/1127565
> Commit-Queue: Takuto Ikuta <tikuta@chromium.org>
> Reviewed-by: Eric Stevenson <estevenson@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#573145}

Bug: 810298, 860251
Change-Id: Ibd30991d65153553a8c36d1af82552461c85966f
Reviewed-on: https://chromium-review.googlesource.com/1132494
Reviewed-by: agrieve <agrieve@chromium.org>
Commit-Queue: Takuto Ikuta <tikuta@chromium.org>
Cr-Commit-Position: refs/heads/master@{#574029}
1 file changed