recovery: install packages in a known mount environment

When installing a package, we should have /tmp and /cache mounted and
nothing else.  Ensure this is true by explicitly mounting them and
unmounting everything else as the first step of every install.

Also fix an error in the progress bar that crops up when you do
multiple package installs in one instance of recovery.

Change-Id: I4837ed707cb419ddd3d9f6188b6355ba1bcfe2b2
diff --git a/screen_ui.cpp b/screen_ui.cpp
index 6a63858..8376341 100644
--- a/screen_ui.cpp
+++ b/screen_ui.cpp
@@ -467,10 +467,11 @@
     if (progressBarType != type) {
         progressBarType = type;
-        update_progress_locked();
     progressScopeStart = 0;
+    progressScopeSize = 0;
     progress = 0;
+    update_progress_locked();