java_sdk_library: Extract child module naming to common struct

Extracts the child module naming to the commonToSdkLibraryAndImport
struct so it can be reused by both sdk library source and prebuilt.

Also:
* Makes all child module name generation go through the common struct.
* Renames some of the methods to clarify their purpose and improve
  consistency.

Test: m nothing
Bug: 155480189
Change-Id: I9be9aab07252b491ab292b8847138a93b487cf53