[Android WebView] Remove WebView-specific "exceptional" paths list

[Android WebView] Remove WebView-specific "exceptional" paths list

1. Remove WebView-specific "prune paths", as all of them are now in
   Chromium-wide PRUNE_PATHS list from licenses.py
   (https://codereview.chromium.org/1153763006)

2. Extract "FindThirdPartyDirsWithFiles" in licenses.py, which can
   also be used in webview_licenses.py

3. Remove known_issues.py, as this was the list of dirs that are
   "incompatibly licensed" w.r.t. Android tree (in fact, we could remove
   this list earlier, when I was removing the "incompatible directories"
   list generator, but I was misguided by the fact that this list was
   also used in other code, although its effect on that code is actually
   void).

BUG=486158

Review URL: https://codereview.chromium.org/1155163007

Cr-Commit-Position: refs/heads/master@{#331792}