Add @hide javadoc to @CorePlatformApi(status = STABLE) and private constructors

It prepares the javadoc ready for adding @SystemApi which requires
hidden APIs documented as @hide.

@SystemApi adds default constructors into its API set, and thus
private constructors are added to prevent this unintended exposure.

Bug: 157639992
Test: m droid
Test: Verified that no change in mmodules/intracoreapi/api/.
Change-Id: Iff5863d5eb45d49b109c88f746dd7bb23b9f9d12
42 files changed