Support to package prebuilt NDK libstlport_shared.so into apk

To package libstlport_shared.so, use the following syntax in the app
module definition:
LOCAL_NDK_STL_VARIANT := stlport_shared
LOCAL_NDK_VERSION := <num>

The rationale behind is:
The NDK prebiult libstlport_shared.so is intentionally different from
the system libstlport.so.
Essentially they are slightly different versions of the library so
shouldn't be mixed up.
libstlport.so is used by the system, but not exposed by the NDK, it is
routinely tweaked in various ways between platform releases, and there
is no guarantee that its ABI is stable, or that it will not be replaced
by something else in the future.
libstlport_shared.so is typically distributed with application packages,
and should not be part of the system.

Change-Id: I72c9911781f7d8402687e9891f3f605f3a60be22