Depend on the more specific ndk timestamp.

There are now two NDK sysroot timestamp files: ndk_base which depends
on the headers and stub libraries, and ndk which depends on ndk_base
and the static libraries. For building modules that need the NDK, we
only need to depend on the base sysroot.

Test: make native
Bug: None
Change-Id: I803032eda125a632d780c9cde10f47270491d0d2