Define __ANDROID_API__ when building for the NDK.

Traditionally this has come from android/api-level.h, but with the
libc headers unified it must be set by the build system since we don't
have per-API level copies of that header now.

Test: make checkbuild with other libc ndk_library patches
Change-Id: Idf6cbba131f065b048b1b412e992c55e3d17e701