[BUGFIX] Fix temporary path for AvailableProxyClasses.php (Revision 2)

This fixes the temporary path used for "AvailableProxyClasses.php" for
situations when a application sub context was used.

Provided that the context is "Development/MyMachine", without this patch
Flow would create a file
"Data/Temporary/Development/MyMachine/AvailableProxyClasses.php" while
all other temporary files would reside in
"Data/Temporary/Development/SubContextMyMachine/".

With this patch, the path will be
"Data/Temporary/Development/SubContextMyMachine/AvailableProxyClasses.php"

Change-Id: I1d24613164ec59ad8701da8a13abb35f77b94fe0
Releases: 2.2, master
Revision-Id: 9c791399d7481d8b14e0180922277eaa370bdf29