Add LLNDK liblog stub library for the VNDK

This replaces the liblog_vndk_headers with a true stub library that
exports the vndk headers, and exports the vndk-specific set of symbols.

Test: generated NDK stub maps are the same before/after
Test: aosp_arm; m -j
Test: Enable BOARD_VNDK_VERSION on aosp_arm; m -j (with other changes)
Change-Id: I9037241963b3872a3bc7cf7ab36a70dd4ddcf9bc