webview_zygote needs to preload the WebView implementation, which may be
an installed APK, so must be able to read and execute code from inside
the APK.

Also add additional neverallow assertions to strengthen some
restrictions on this domain.

Test: WebView apps work after installing a WebView APK.
Bug: 21643067
Change-Id: I58aedc5e0a25259e2e20c70d4260579a354b6789

Test: m
Test: angler boots

Bug: 21643067
Change-Id: I7530120385766506405a318caac8ef143e71cce7

The webview_zygote is a new unprivileged zygote and has its own sockets for
listening to fork requests. However the webview_zygote does not run as root
(though it does require certain capabilities) and only allows dyntransition to
the isolated_app domain.

Test: m
Test: angler boots

Bug: 21643067
Change-Id: I89a72ffe6dcb983c4a44048518efd7efb7ed8e83