Implement resource merging for flavored source sets

[arunkumar9t2]

This PR introduces new API in the previously introduced unified macro #87 for resource pre-processing.

In AGP land, we can have multiple resource directories for a variant and before calling aapt AGP would merge these sources based on priority order BuildType -> Flavor -> main -> Dependencies. This PR tries to implement the same functionality by introducing a new tool RESOURCE_MERGER and a new android rule API resources to specify multiple source sets. Internally merging is performed using same code from the android_tools package that bazel uses and then the result is passed to resource_files as usual.

The structure lays foundation for worker support, asset and manifest merging support later on.

Implementation notes

• Backwards compatible with resource_files API
• Sorting order is used to denote priority of directories
• tools/aapt_lite/src/main/java/com/google/devtools/build/android/ResourceMerger.java is in Java since android_tools had trouble getting added as Kotlin dependency
• Each resource directory accepts a dict so as to pass additional information like manifests in the future.
• Only directories are accepted
• Expand locations and make variables are not supported yet.

Example usage

android_binary(
    name = "sample-android-flavor1-paid-debug",
    srcs = glob([
        "src/main/java/com/grab/grazel/android/sample/MainActivity.kt",
    ]),
    assets = glob([
        "src/main/assets/assert-file.png",
    ]),
    assets_dir = "src/main/assets",
    custom_package = "com.grab.grazel.android.sample",
    resources = {
        "src/main/res-debug": {},
        "src/paid/res": {},
        "src/main/res": {},
    },
)