aconfig: restructure integration tests

Consolidate integration tests under tests/. Rename build targets to
follow the same pattern.

The important aconfig build targets are:

  - aconfig           : the aconfig binary
  - aconfig.test      : the aconfig unit tests
  - aconfig.test.java : the Java integration tests
  - aconfig.test.cpp  : the C++ integration tests
  - aconfig.test.rust : the Rust integration tests (not implemented yet)

Note: the Rust integration tests are blocked until the build system has
added support for aconfig Rust libraries.

Note: the flags used in the integration are not set correctly. A
follow-up CL will fix this.

Bug: 283911467
Test: atest aconfig.test.java aconfig.test.cpp
Change-Id: Idb8dad4c38a49231c2e89228775bacf065ee1f57