Build Android extension with one-cmake-pass

[kirklandsign]

🚀 The feature, motivation and pitch

Right now we need to build android extension with two cmake passes: one on main ET, one on android specific extension. See https://github.com/pytorch/executorch/blob/main/build/build_android_llm_demo.sh#L38-L81

Ideally, we could make one pass. This way, it is also easier to build the AAR.

Alternatives

No response

Additional context

No response

RFC (Optional)

No response

[kirklandsign]

Current blocker: name collision within XNNPACK and absl

CMake Error at extension/llm/tokenizers/third-party/abseil-cpp/CMake/AbseilHelpers.cmake:321 (add_library):
  add_library cannot create target "memory" because another target with the
  same name already exists.  The existing target is created in source
  directory "/Users/hsz/executorch/backends/xnnpack/third-party/XNNPACK".
  See documentation for policy CMP0002 for more details.
Call Stack (most recent call first):
  extension/llm/tokenizers/third-party/abseil-cpp/absl/memory/CMakeLists.txt:17 (absl_cc_library)