Split core platform API into Conscrypt, ART, I18N modules

Adds droidstubs and java_library definitions to generate the stubs
sources and compile them for each of the art, conscrypt and i18n
modules. Also adds a java_system_modules definition for art module
core platform api stubs to allow it to be used to build the i18n and
conscrypt libraries.

Removes generation of core-platform-api-stubs and instead builds
core.platform.api.stubs from the module specific libraries.

The art module reuses the API specification files (e.g.
mmodules/core_platform_api/api/platform/current-api.txt, the other
modules have their own directory.

The changes have been made in here to make reviewing easier. However,
the intent is that the i18n and conscrypt build rules and API
specification files will be moved to external/icu and
external/conscrypt respectively in a follow up change.

Bug: 142045649
Test: m checkbuild
Change-Id: I2502c80967c2e3a0f91cd09c65fb54e609c05d4c
11 files changed