Mitigate the races during installation.

During the installation of update, the application would be killed
twice. The first one is done by PackageFreezer. It kills and freezes
the application while starting installing. The second one happens
just after the package is unfrozen. Since the application won't be
able to be started when it's frozen. The second kill is a little
overkill to me. It's possible that someone starts the application
before the second kill comes. Skip the second kill to mitigate the
possible race.

Bug: 165012101
Test: atest PackageManagerTest
Change-Id: I5085547ba3f20c370474228deb13bbc24064ae21