Allow webview_zygote to read/execute installed APKs.

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