Tweaks to download manager API based on API review feedback

* improved documentation of the default destination on the download
  cache and the concomitant consequences

* removed NETWORK_WIMAX flag for setting allowed networks

* changed request headers behavior to support multiple instances of
  the same header (as allowed in the HTTP spec), renamed
  setRequestHeader() to addRequestHeader()

* accept user-facing strings as CharSequences instead of Strings

* new convenience methods setDestinationInExternalFilesDir() and
  setDestinationInExternalPublicDir() for setting a destination in
  either shared or app-private external storage directories

* renamed setMediaType() to setMimeType()

Change-Id: I8781e2214d939c340209cab917bbbba264ab919c