Export libui_headers as vndk header only library

Emulator EGL uses libui headers and needs to be built
successfully when BOARD_VNDK_VERSION is enabled.

Bug: 64704866
Test: Built aosp_x86 with BOARD_VNDK_VERSION set to current and
      BOARD_VNDK_RUNTIME_DISABLE set to true. Booted on x86 emulator.

Change-Id: Ic26fc14e61330fcc555501c906a64393afe6145d