Reland "Make java_checker.py read files in parallel"

This is a reland of commit 0eb200c0678d7622a855d50bfa379a1be126cd75

java_files should have been an absolute path to the file, and not
just the file name.

Original change's description:
> Make java_checker.py read files in parallel
>
> This speeds up Java presubmits especially on filesystems where
> reading a file is slow.
>
> On cog, this change approximately halves the time to run
> CheckUnwantedDependencies for a Java change.
>
> Bug: b/358457858
> Change-Id: Iedd9b289c0b2c1029a7c4c937f45873d3c5f5c5a
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5798514
> Reviewed-by: Peter Wen <wnwen@chromium.org>
> Commit-Queue: Gavin Mak <gavinmak@google.com>
> Reviewed-by: Takuto Ikuta <tikuta@chromium.org>
> Cr-Commit-Position: refs/heads/main@{#1346770}

Bug: b/358457858
Change-Id: I8a7e81dcb6d2bcdcb34f928e0af3e4111de65a4d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5824269
Commit-Queue: Gavin Mak <gavinmak@google.com>
Reviewed-by: Takuto Ikuta <tikuta@chromium.org>
Reviewed-by: Peter Wen <wnwen@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1350270}