Split libnbaio into libnbaio + libnbaio_mono.

When the vndk is enabled (BOARD_VNDK_VERSION=current),
vendor libraries must only link against vendor variants
in the build system. This also means that they receive restricted
sets of headers and that automatic global headers are restricted.

Test: mma -j BOARD_VNDK_VERSION=current
Test: boots device, audio plays over YouTube
Bug: 37342627
Bug: 63269228
Change-Id: Ifa9c96f6b98b456def485be6bc5046ee54c3c3ca