Separate modules for API docs and API stubs

frameworks/base/Android.bp became too big. Modularize it into multiple
files for better readability and extensibility.

droidstubs and java_library modules for API stub libraries are moved to
StubLibraries.bp.

droidstubs and droiddoc modules for API documents are moved to
ApiDocs.bp

Also, added some documentation to explain the roles of the many modules
and how they are related with each other.

Bug: N/A
Test: m
Change-Id: I285e8c82078d5bd3ed85b0343db2f6375ce7ffb7