Mount all partitions before running postinstall

For Virtual AB Compression, we need to mount snapshotted partitions
manually.

Test: treehugger
Change-Id: I15af8375e393ce2539d2075b467e8dbe94bec247
diff --git a/common/error_code.h b/common/error_code.h
index 7d9cfff..a889888 100644
--- a/common/error_code.h
+++ b/common/error_code.h
@@ -86,6 +86,7 @@
   kNotEnoughSpace = 60,
   kDeviceCorrupted = 61,
   kPackageExcludedFromUpdate = 62,
+  kPostInstallMountError = 63,
 
   // VERY IMPORTANT! When adding new error codes:
   //