[Android WebView] Clean up resources loading logic
[Android WebView] Clean up resources loading logic

Use ui/base code added in http://crrev.com/335964 and http://crrev.com/336330
instead of our custom code.

This also allows retrieving pak files' fd and region via GetMainAndroidPackFd
and GetLocalePackFd functions later, which we will need for passing the
resources to child services.

BUG=522205

Review URL: https://codereview.chromium.org/1303093002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#344785}

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