generate_gradle.py: Fix --target //chrome/android:chrome_public_test_apk

There were two issues:
1) The script was not building the .build_config.json files when
   --target is passed,
2) The _test_apk suffix needed to be updated.

Bug: 1213000
Change-Id: Ib8f170e567adbc43d0b2bb2d219a3499c85b76d4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4702027
Commit-Queue: Peter Wen <wnwen@chromium.org>
Reviewed-by: Peter Wen <wnwen@chromium.org>
Auto-Submit: Andrew Grieve <agrieve@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1172474}