android: fix unreadable text in doc browser's about dialog

There were two inconsistencies here:

- The doc browser activity had an explicit android:theme, but the viewer
  does not, so remove it to be consistent, this fixes the about dialog's
  "black text on black background" problem.
- Now the doc browser had white text on white background: the about
  dialog explicitly uses android:textColor for the strings which should
  be readable on a bright background, do the same in
  file_explorer_folder_icon and file_explorer_grid_item, that makes doc
  browser text readable as well.

Change-Id: Iae9103556ca7c32c9ac8ea02d9e5d5fa6dfb72b5