support "sideload over ADB" mode

Rather than depending on the existence of some place to store a file
that is accessible to users on an an unbootable device (eg, a physical
sdcard, external USB drive, etc.), add support for sideloading
packages sent to the device with adb.

This change adds a "minimal adbd" which supports nothing but receiving
a package over adb (with the "adb sideload" command) and storing it to
a fixed filename in the /tmp ramdisk, from where it can be verified
and sideloaded in the usual way.  This should be leave available even
on locked user-build devices.

The user can select "apply package from ADB" from the recovery menu,
which starts minimal-adb mode (shutting down any real adbd that may be
running).  Once minimal-adb has received a package it exits
(restarting real adbd if appropriate) and then verification and
installation of the received package proceeds.

Change-Id: I6fe13161ca064a98d06fa32104e1f432826582f5

default_device.cpp

@@ -26,8 +26,7 @@ static const char* HEADERS[] = { "Volume up/down to move highlight;",
                                  NULL };
 
 static const char* ITEMS[] =  {"reboot system now",
-                               "apply update from external storage",
-                               "apply update from cache",
+                               "apply update from ADB",
                                "wipe data/factory reset",
                                "wipe cache partition",
                                NULL };
@@ -72,10 +71,9 @@ class DefaultDevice : public Device {
     BuiltinAction InvokeMenuItem(int menu_position) {
         switch (menu_position) {
           case 0: return REBOOT;
-          case 1: return APPLY_EXT;
-          case 2: return APPLY_CACHE;
-          case 3: return WIPE_DATA;
-          case 4: return WIPE_CACHE;
+          case 1: return APPLY_ADB_SIDELOAD;
+          case 2: return WIPE_DATA;
+          case 3: return WIPE_CACHE;
           default: return NO_ACTION;
         }
     }