Add option to automatically unmount system before installing ZIP

This adds the option to automatically unmount the system partition
before installing a ZIP-file.
This option is enabled by default, This is usefull on SAR-devices
where system is mounted as system_root and ZIPs (i.e. Magisk)
try to unmount system instead of system_root and fail to install.

Change-Id: Id94ae658964679e23aa7fa137e07edf74ee073df

data.cpp

@@ -783,6 +783,7 @@ void DataManager::SetDefaultValues()
 #else
 	mPersist.SetValue(TW_NO_SHA2, "1");
 #endif
+	mPersist.SetValue(TW_UNMOUNT_SYSTEM, "1");
 
 #ifdef TW_NO_SCREEN_TIMEOUT
 	mConst.SetValue("tw_screen_timeout_secs", "0");