Add UI for mainline modules licenses.

Added a module licenses option that lives in Legal information settings.
Clicking that option opens module licenses page, which displays every
module by name, filtered to exclude modules without license files.
Clicking a module in the list opens HTMLViewer.

Created ModuleLicensesProvider, a new ContentProvider that serves as a
redirect for the Uris sent to HTMLViewer so that they open asset files.
In order to provide the redirect, the provider will write the license file
to a file in Settings' cache directory when the license does not exist
in the cache or is outdated. The provider then opens that cached file.

Fixes: 135183006
Test: robotests
Change-Id: I7d69da34780c8c4efb150d0c0411078c12bc80d8