Two file accesses file.exists() && file.lastModified() can be replaced
by single file access Files.getLastModifiedTime()

Change-Id: I9746f6e00f13a113ffec5c2ee94f2fa9a46ccbae
Signed-off-by: Joerg Kubitz <jkubitz-eclipse@gmx.de>
Reviewed-on: https://git.eclipse.org/r/c/pde/eclipse.pde.ui/+/185821
Tested-by: PDE Bot <pde-bot@eclipse.org>
Tested-by: Lars Vogel <Lars.Vogel@vogella.com>
Reviewed-by: Lars Vogel <Lars.Vogel@vogella.com>