aconfig: add java codegen test mode

Add java codegen test mode. The test mode will generate Flags.java and
FeatureFlagsImpl.java differently.
    * Flags.java will have getter and setter function to switch the
      FeatureFlagsImpl. Flags.java will not initialize the instance
      of FeatureFlagsImpl during initialization, thus it will force the
      user to set up the flag values for the tests.
    * FeatureFlagsImpl removes the dependency on DeviceConfig, and
      allows the caller to set the values of flags.

Command changes
This change adds a new parameter `mode` to `create-java-lib` subcommand.
The default value of `mode` is production, which will generate files for
production usage, and keeps the same behavior as before.

The new `mode` test is added to trigger the test mode. The command is
aconfig create-java-lib --cache=<path_to_cache> --out=<out_path>
--mode=test

Test: atest aconfig.test
Bug: 288632682
Change-Id: I7566464eb762f3107142fe787f56b17f5be631b7