WebView: Update setAllowFileAccess doc to reflect new default value

Update setAllowFileAccess java docs to reflect that it's now disabled by
default after merging crrev.com/c/2056824. Also add a note to use
androidx WebViewAssetLoader instead.

Bug: 148840827
Test: m offline-sdk-docs -j20
Change-Id: I15866ab63818771fd91f40828846d6b4c39d278e