Refactor java/droiddoc.go in Soong.

The GenerateAndroidBuildActions() is too big to be maintained and added-in new
features.

Since the GenerateAndroidBuildActions() is a giant code block having build flags
setting and workflow logic mixed in it. So seperate these two main
stuff to only keep workflow logic in the function, and create other
indipendent flags setting functions to increase the modularity.

Test: m -j api-stubs-docs && m -j metalava-api-stubs-docs, and check
build.ninja file
Bug: b/70351683

Change-Id: I4230d353756aeb4b24640f641b8f8bab6b21204d