Add @NotNull, @Nullable, and @IntDef/@StringDef annotations

This changeset adds some annotations for recording whether a
method return value or method parameter can be null (@Nullable),
can never be null (@NotNull), or must be an integer enum or flag
using one of the given constants.

Change-Id: Ic932592ea3bac781c1df364447c57042461333c5
5 files changed