sync: Add NDK sync.h and libsync

The new header provides an updated interface to libsync appropriate
for the NDK. Clients use existing syscalls where possible (e.g. poll()
instead of sync_wait()), and the remaining functions return structures
used in mainline Linux kernels rather than the Android staging sync
framework.

Over time, framework clients will be migrated to using the NDK
interface, which will eventually replace the current internal
interface. The only difference is the header will be named
<android/sync.h> in the NDK and <sync/sync.h> internally.

Bug: 35138793
Test: sync-unit-tests on bullhead
Change-Id: Ieb3649b80565393e26b604416158438d32c2a256