Add support to continue download when HTTP response etag is empty

As per AOSP behaviour application download can not resume when
HTTP response header contains empty etag.

Add fake Etag to support continue download in HTTP server
response header.

Change-Id: Iea7a51b3ada04cffe5f15253dbf3bec038d7146d
CRs-Fixed: 623780

Conflicts:
	src/com/android/providers/downloads/DownloadThread.java
change to make new drawables take effect

Change-Id: I6439d150f3b73c5ce766c64491513dc09c7be964