[code coverage] Merge Java coverage files to avoid duplicate names

Example merge script failures on bot:
https://ci.chromium.org/p/chromium/builders/ci/android-code-coverage/1633
It also conforms to JUnit tests as JUnit tests generate <junit_test_target_name>.exec file

Bug: 972171
Change-Id: Id1e34e0146f164540d95e8ec8377a1623f7eda57
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1682549
Commit-Queue: Yun Liu <yliuyliu@google.com>
Reviewed-by: John Budorick <jbudorick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#673785}