[Android] Expand Java GN rules to support more complex usecases

[austinh0]

Problem

• Our Java GN rules are very simple and don't support things such as a java_library depending on another java_library
• In the next PR, will use Gradle to download JARs that we want to depend on, and need a GN rule to wrap around those "prebuilt" jars.

Change overview

• The following changes are loosely based on Chromium (write_build_config, java_library_impl).
• Add jar_path field in java_library template, mutually exclusive with sources. If set, the build rule skips the javac and jar actions and instead just copies the JAR from jar_path into the build directory.
• Create write_build_config.py script, which outputs a JSON file ("build config") describing each build target.
  • Specifically, contains list of dependent build configs, and dependent JARs (recursive) for the target.
  • This allows us know about child JARs when compiling a parent build rule, and add them to the classpath.
• Amend javac_runner.py to read a build config JSON and modify classpath accordingly.

Testing

• Add python test, which checks that the correct JARs are created, and that the contents of build configs match expected.
  • Caveat: in order to test java_prebuilt rules, I created generate_jars_for_test.py. This script needs to be run before other Java things are built, or GN won't be able to find the jar_path.
• In android.py, go back to copying JARs to libs/. The app only needsCHIPController.jar and SetupPayload.jar, but the chipSdkBuildDir approach would copy all JARs.

[austinh0]

@chrisdecenzo @Damian-Nordic @hawk248 @jmartinez-silabs @msandstedt?