libbacktrace is VNDK-SP

libbacktrace and its dependents are now VNDK-SP. Since they are in
/vendor/lib/vndk-sp directory, they are accessible directly from the
vndk namespace. Therefore, libbacktrace is removed from the exported
shared libs list of the default namespace.

Bug: 37413104
Test: sailfish builds and boots
Merged-In: I137c17d55940b783eab6d0125bc4d26b96bcc2f2
Change-Id: I137c17d55940b783eab6d0125bc4d26b96bcc2f2
(cherry picked from commit b2a4b8cd)