Add fallback packages to be enabled iff no webview packages are valid

This patch makes it possible to declare a WebView package as a fallback
which means that the package will be enabled iff there exist no other
valid and enabled (and available-by-default) webview packages.

The enabled-state of a fallback package is updated at boot and if a
webview package is changed (it it's been up/downgraded or has had its
enabled-state changed).

This patch also adds 'webviewupdate' shell commands for enabling and
disabling this mechanism.

Bug: 26375524, 26375860
Change-Id: I151915e5d6d932697dab10aeb593687e6b9c817e