Separate output of flag generation from hiddenAPIFlagFileInfo

HiddenAPIFlagOutput encapsulates the paths to the files produced by the
hidden API flag generation of a single bootclasspath_fragment. It is
returned from hidden API flag generation and is embedded within the
hiddenAPIFlagFileInfo so they can be passed to other modules.

Unlike the fields it replaces in hiddenAPIFlagFileInfo the fields in
HiddenAPIFlagOutput are of type Path not Paths which makes it easier to
use.

Bug: 179354495
Test: m com.android.art com.android.ipsec com.android.os.statsd com.android.conscrypt
      - verify that this does not change the contents of the apex files
Change-Id: I7373ba1001cac3a75eb54a23e62fa52f5013ee7f