Do not read 'vendor_available: false'

In case of VNDK, 'vendor_available: false' had a special meaning that
defines VNDK-private libraries. It is not trivial because not
defining a boolean property means 'false' normally. To avoid the
confusion replace it with the 'vndk.private: true' for VNDK-private
libraries and 'private: true' for LLNDK-private libraries.

All VNDK libraries must define 'vendor_available: true' and may have
'vndk.private: true' if they are VNDK-private.
With this change '(vendor|product)_available: false' is the same as
not defining the property.

LLNDK-private must define 'private: true' instead of
'vendor_available: false'.

Bug: 175768895
Test: build
Change-Id: I57fbca351be317257d95027f3cdcdbbe537eab23