Add vndk tag for libunwindstack which must be a vndk-sp

As a VNDK-SP module, Android.bp must have 'vndk' tag as well as
'vendor_available: true'.

The 'vndk' tag for VNDK-SP module is formated as below:
vndk: {
    enabled: true,
    support_system_process: true,
},

VNDK-SP modules will be installed both in system/lib(64) as normal and
in system/lib(64)/vndk-sp as a vendor variant.

Bug: 63866913
Test: build with BOARD_VNDK_VERSION=current
Change-Id: Idb4757988d1799ffcd3341343fcc4dac34bf7ef3
(cherry picked from commit 14d540a1)