[HTMLViewer] Explicitly set WebSettings#setAllowFileAccess to true

WebSettings#setAllowFileAccess default value is changing to be off by
default for Apps targetting API >= 30. This explicitly turns on this
setting for the HTMLViewer app so it can load local files.

Bug: 148840827
Test: N/A
Change-Id: I632cec2af75e90d097e86081a6379afdbdf66162
(cherry picked from commit 57ab31bd6df1dda5c502cef163cb5a7ae3abe71f)