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: Ifb385d907018bbe76b2aa81eb09209a77ea9224d