aconfig: add Java integration tests

Add integration tests for Java. This test setup verifies that

  - the build system calls aconfig to generate a Java library
  - the Java test compiles against the auto-generated library
  - the auto-generated code returns expected values

Similar integration tests for C++ and Rust will be added in follow-up
CLs.

Note: the build does not currently support specifying that
tests/*.values should be applied, so the test flags will all be assigned
the defaults. A later CL will fix this.

Bug: b/283911467
Test: atest aconfig.test aconfig.test.java
Change-Id: Ia365e209261f4935a23e2dac9ef0ab5b60f76e52