Android: Fix dex.py zipalign computation to actually work.

Messed up the math between testing & code review tweaks. :/

Bug: 981974
Change-Id: Iad2151405c539b52d39ba3c5ba9f78ac149aec7b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1777090
Commit-Queue: Andrew Grieve <agrieve@chromium.org>
Commit-Queue: Sam Maier <smaier@chromium.org>
Auto-Submit: Andrew Grieve <agrieve@chromium.org>
Reviewed-by: Sam Maier <smaier@chromium.org>
Cr-Commit-Position: refs/heads/master@{#691918}

build/android/gyp/util/zipalign.py

@@ -69,7 +69,7 @@ def _SetAlignment(zip_obj, zip_info, alignment):
   """
   cur_offset = zip_obj.fp.tell()
   header_size = _FIXED_ZIP_HEADER_LEN + len(zip_info.filename)
-  padding_needed = (header_size - cur_offset) % alignment
+  padding_needed = (cur_offset - header_size) % alignment
 
   # Extra field used to 4-byte align classes.dex. Alignment speeds up
   # execution when dex files are used via incremental install.