DO NOT MERGE: Add support to send notification from Zygote to system_server

Zygote will send message to system_server on SIGCHILD now.

Bug: 136036078
Test: atest CtsWebkitTestCases
Test: atest CtsHostsideWebViewTests
Test: atest CtsAppTestCases
Change-Id: I0ad2a3299f34344a12e2a3c153ca518aca6f6f11
Merged-In: Ifb385d907018bbe76b2aa81eb09209a77ea9224d