tools/progress: Add finish and abort functions

We have external progress handlers which would like to implement these
functions, in order to update the screen upon completion or error.

Change-Id: I2df4654d5e2092407a83d2c8bbb020dcce84ac50
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/tools/p2a.cpp b/tools/p2a.cpp
index 090b9bd..b92887f 100644
--- a/tools/p2a.cpp
+++ b/tools/p2a.cpp
@@ -128,6 +128,7 @@
         }
     } while (bytesRead > 0);
 
+    progress->finish();
     return true;
 }