Allow NDK static libraries to use the NDK sysroot.

Building a static library for the NDK only requires that the NDK
headers be available. Currently, A module with both
`static_ndk_lib: true` and `sdk_version: something` will have a cyclic
dependency since it both needs the NDK and is in the NDK. Create two
NDK timestamp files: one for the isolated parts of the NDK (headers
and stub libraries), and another for the full sysroot with the static
libraries.

Test: set static_ndk_lib on compiler-rt-extras, make ndk
Bug: None
Change-Id: Iab50ffa0e4cbf4cd164f376e15281030c7aad984