Android: Avoid name conflicts for __unpack_aar

Some targets only differ in their names by the number of _java. e.g.
androidx_window_window_java and androidx_window_window_java_java. Thus
fixing the replacement of _java with _J rather than empty string to
distinguish between the two.

Example failure: https://crrev.com/c/3008075

Bug: None
Change-Id: Icda2fd502dc7142dc7801228ece69817b355fa79
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3009075
Commit-Queue: Peter Wen <wnwen@chromium.org>
Owners-Override: Peter Wen <wnwen@chromium.org>
Auto-Submit: Peter Wen <wnwen@chromium.org>
Reviewed-by: Mohamed Heikal <mheikal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#898858}