Fix outstanding issues with conversion to NIO

NIO methods throw NoSuchFileException, not FileNotFoundException. How
nice.

Throwing from Files#getLastModifiedTime, apart from cluttering up
throws clauses, means callers have to check for FileNotFoundException,
which is verbose and error prone. Instead, implement a helper with the
old File#lastModified() semantics; this preserves old behavior, which
might not handle some error cases well, but worked well enough. (In
theory we could use FileSnapshot or something, but I don't want to
shave that particular yak until/unless I get a chance to add a
Path-compatible interface to FileSnapshot in JGit as well.)

Organize some imports.

Tested:
-Tests pass.
-Initialized a site, including downloading.
-Launched a site.
-Added and removed site headers and CSS in a running site.
-Added and removed plugins, both live and while shutdown.

Change-Id: I60dfe5280512f00dce4ec99fe3689b2f2b3c6685

gerrit-pgm/src/test/java/com/google/gerrit/pgm/init/UpgradeFrom2_0_xTest.java

@@ -15,7 +15,6 @@
 package com.google.gerrit.pgm.init;
 
 import static java.nio.charset.StandardCharsets.UTF_8;
-
 import static org.easymock.EasyMock.createStrictMock;
 import static org.easymock.EasyMock.eq;
 import static org.easymock.EasyMock.expect;