libprocessgroup: Add sendSignalToProcessGroup

Add a function which sends signals to all members of a process group,
but does not wait for the processes to exit, or for the associated
cgroup to be removed.

Bug: 274646058
Ignore-AOSP-First: Dependency of ActivityManager change which developed on interal git_master
Test: Force-stop of chrome with 15 tabs completes ~500ms faster
Test: Full Play store update causes no ANR
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:d87b6018d25cbbd33b345dc58c634718bf5d0def)
Merged-In: I37dbdecb3394101abbee8495e71f6912b3c031f5
Change-Id: I37dbdecb3394101abbee8495e71f6912b3c031f5