Revert "Remove libstlport from PRODUCT_PACKAGES."

The platform dependence on stlport is now far enough gone that a
regular build no longer builds stlport (woot!). Unfortunately, vendor
blobs don't claim their dependencies, so the build system has to keep
track of this for them.

Bug: 18777920
This reverts commit ef11722e.

Change-Id: I6a0dd26dfc6837c419eb5dd2ec5258dd323fd4bf