Split the core/platform API into stable and legacy versions.

This splits all the targets under libcore, based on an attribute on
the @CorePlatformApi annotation. Everything outside libcore still uses
the legacy versions for now. The ICU and conscrypt targets need to be
split in a similar fashion.

Test: treehugger
Bug: 157640067
Change-Id: If3e03c2e87e19a5b3584d04f2b59642edff17965

Change-Id: I2fd62fdc5cedbfbbe6bb06277dad79363ace4ff8
12 files changed