Extract common stub flags code

The stubFlagsRule does three separate tasks:
1. It computes the set of modules that provide the stubs.
2. It scans all the modules to find the stub modules and retrieves the
   paths to their dex files.
3. It constructs the ninja rule.

Of those three tasks, 1 and 3 will be same for the
platform_bootclasspath. Instead of searching all the modules for the
ones that provide the stubs it will simply add dependencies onto the
stub modules and retrieve the dex file paths from them.

This change extracts tasks 1 and 3 into separate methods for reuse. It
also parameterizes the generation of the ninja rule.

Bug: 179354495
Test: verified that the monolithic out/soong/hiddenapi/... files are
      unchanged by this change
Change-Id: I893845dbddc4b001dfd44d0e0b1c8a31b7f3f89f