Add vendor_available to liblog_headers.

Renamed NDK headers to "liblog_ndk_headers"
(these names aren't used anywhere).

libutils_headers now properly export liblog_headers.

Test: with BOARD_VNDK_VERSION=current
Change-Id: I7b2d9e77e313e3f2ef362a19a03f4c23874d589a
Merged-In: I3a85385f588b84393c57fd6d1bcac620f708f0f1