Add TWRP app install via TWRP recovery

Note: I will have to add a build flag for excluding the app later for
watches and maybe Android TV. I will also have to add support in the
tablet and watch layouts later. I will merge this for the initial roll out
to select devices and finish up later.

Change-Id: Ia4ce5522fae542afa1539b10c0691315392a19ab
diff --git a/twrp-functions.cpp b/twrp-functions.cpp
index 9b0356f..92d3a3b 100644
--- a/twrp-functions.cpp
+++ b/twrp-functions.cpp
@@ -788,7 +788,10 @@
 	string propvalue;
 	if (!PartitionManager.Mount_By_Path("/system", true))
 		return propvalue;
-	if (TWFunc::read_file("/system/build.prop", buildprop) != 0) {
+	string prop_file = "/system/build.prop";
+	if (!TWFunc::Path_Exists(prop_file))
+		prop_file = "/system/system/build.prop"; // for devices with system as a root file system (e.g. Pixel)
+	if (TWFunc::read_file(prop_file, buildprop) != 0) {
 		LOGINFO("Unable to open /system/build.prop for getting '%s'.\n", Prop_Name.c_str());
 		DataManager::SetValue(TW_BACKUP_NAME, Get_Current_Date());
 		if (!mount_state)