Wait for RELRO before starting WebViewZygote at boot.

We start WebViewZygote at boot time to ensure the first app to use
WebView doesn't have to wait, but we were not waiting for RELRO creation
to finish before doing so. Move the background task which starts the
zygote to WebViewUpdateServiceImpl and have it wait for the RELRO first
to ensure that we get the memory saving of RELRO sharing whenever
possible, instead of only when the timing happens to work out by chance.

Fixes: 130305037
Test: manual, check logs and relro sharing status after boot
Change-Id: I55c3f80b0db1dc82727b90c70f777618ca77a942