Mark libtinyxml2 vendor_available.

libtinyxml2 belongs to vndk-cap. Mark it vendor_available to enable vndk
abi stability checks on it.

Details: https://android-review.googlesource.com/368372

Test: mm -j64

Bug: 38244611

Change-Id: Ide7ea89ae22037b75fd70d6238c76b76b5d1a923