Make WebViewFactory more robust.

Catch and discard any exception thrown in getWebViewNativeLibraryPaths
to avoid the system server crashing if there is something wrong with the
update APK. prepareWebViewInSystemServer(nativePaths) is safe to call
even if nativePaths is null or invalid, and must be called to ensure
that any process waiting for relro processing is unblocked.

Bug: 16894062
Change-Id: I76c759bebcb7c4643fb50979376afea764c859c4