This CL fixes a number of issses in metalava:

(1) Support separate stub file generation for android.jar and for
    documentation stubs. The "--stubs" flag continues to create stubs
    intended for compilation into android.jar, and the new
    "--doc-stubs" flag generates stubs intended for documentation
    migration.

    Why are these separate, you ask? It turns out the stubs we compile
    for android.jar needs to be slightly different from those we
    generate documentation for. Here are some specific examples:

    (a) In the SDK stubs we'll use different nullness annotations for
        recently annotated elements than for elements that have been
        annotated for a while. This allows the Kotlin compiler to
        treat violations of these contracts as a warning instead of an
        error. In the documentation, however, we just want the normal
        @NonNull/@Nullable annotations to show up, not the specialized
        migration versions of these.

    (b) @doconly: Some elements, such as R.styleable, are marked with
        @doconly and should be present when running for example
        javadoc on the code, but should not be present in the actual
        SDK.

(2) This CL updates the set of stub annotations that the stubs can be
    compiled with. These are in many cases nearly identical to the
    androidx.annotations in the support library, but with some notable
    changes, such as allowing type parameter and type use for the
    nullness annotations, switching to class retention for a few
    annotations, and removing some that don't apply.

(3) This CL updates the nullness migration to the new scheme supported
    by Kotlin 1.2.50, and removes earlier handling for multiple stages
    of migration (@Newly<X> vs @Recently<X>).  It also changes the
    annotation package handling from prefering
    android.support.annotation to androidx.annotation.

    There area also various fixes to the nullness coverage code which
    lets the reports be written to files, which more correctly
    accounts for static final primitive constants, etc.

Finally, misc bug fixes I encountered. At this point, metalava can
spit out stubs containing annotations that compile -- and which
contain the right migration names for Kotlin.

Bug: N/A
Test: Unit tests included and updated

Change-Id: I4915e199e2e14473df62206015461108bc58b962