Mark libdemangle as vendor_available.

This means the library may be installed in both the
/system and /vendor partitions, if they are used by
system and vendor code respectively.

Test: mm -j64
Bug: 38244611
Change-Id: I5874529d410a226c028a248b0d22ea5e80639445
Merged-In: I978f8e34d354e4fe03a3c19dd0dbc6899f26a793