libbacktrace is not directly available to vendors

This lib becomes VNDK-SP just because libutils, which is VNDK-SP, is
depending on it. Since libutils provide android::CallStack class for
collecting and printing the callstack, there is no need for vendors to
use the internal libbacktrace library. Thus the lib is marked as
vendor_available: false.

Bug: 64730695
Test: build 2017 pixel device
Change-Id: I4ad91e3a6747bb0ae88d32cd26dfba32239c5b93