webview_packages: prefer the more powerful webview packages

If Gapps are installed, better webview providers
become avaliable.

This commit adds support for those and makes the system
choose the best provider: Chrome Stable>Google Webview>AOSP Webview

It also allows the user to choose between additional
providers from Play Store: Chrome Beta, Chrome Dev and Chrome Canary.

This patch is a collobarative work of:

mfonville <maarten.fonville@gmail.com> @mfonville
rapperskull <rapper.skull@hotmail.it> @rapperskull
Arne-Christian Blystad <arne.christian.blystad@bartec-pixavi.com> @Blystad
Alex Naidis <alex.naidis@linux.com> @TheCrazyLex

Bugfixes and move to an overlay by blunden <blunden2@gmail.com>

You can find more information on this topic here: opengapps/aosp_build#76

Change-Id: I006d814b7c12ffe4f29cd865c9d4cc6a60bc7783