Tweak Intent usage for ChoosePDFActivity

ChoosePDFActivity can be used either to select PDF/XPS etc files,
or to select key files (for digital signatures). The choice of which
one to use is made according to the action string in the Intent with
which the activity is invoked.

Previously we would look for Intent.action.MAIN and take this to mean
"Look for PDF files", and anything else to mean look for key files.
Unfortunately, if you start the activity directly using adb then the
action string is null, so we look for key files.

The fix is to use a specific (custom) string for key files and for
everything else to be treated as a request for PDF files.
Take on curl change from Sebastian.

This should get us better errors out.
Error out on unsupported encrypted zip files
Handle errors from curl by eventually throwing

Previously errors from curl_easy_perform() were never checked. This
caused mupdf to enter an eternal loop, repeatedly trying to get data
to parse from the remote host.