add libvndksupport

libvndksupport is a new LL-NDK library that provides vendor-visible APIs
for platform-only functionalities of other LL-NDK libraries. Currently,
it provides android_(load|unload)_sphal_library which abstracts the
platform-only APIs of libdl (android_get_exported_namespace, etc.)

Bug: 37323945
Test: sailfish builds and boots
Test: libvndksupport-tests passes

Change-Id: I6d2911b57e009d0c842554933aac87d6573ffcbf