Add temporary restriction on hidden API processing

Hidden API processing of a bootclasspath_fragment requires the fragment
provides information, such as dependencies on other fragments and flag
files to override the default flags. Failing to do so will cause hidden
API generation to either fail or to generate different flags to that
generated by the hidden API processing done by platform_bootclasspath
which will cause the build to fail.

Previously, this was handled by only performing hidden API processing
for those modules that provide stub libs and relied on there only being
bootclasspath_fragments defined for ART (which already supports hidden
API processing), and Conscrypt and I18n neither of which provide stubs.
Unfortunately, that can no longer be relied upon due to a couple of
recent changes:
1. A java_sdk_library in a bootclasspath_fragment's content property is
   automatically treated a stub library. That avoids duplication for
   most bootclasspath_fragments that provide the implementation and
   stub libraries through the same java_sdk_library. It does not affect
   either ART, conscrypt or i18n as they all define the implementation
   separately to the stubs.

2. bootclasspath_fragment modules have been defined for a number of
   android modules as they are needed for reasons other than hidden API
   processing.

In combination this meant that rules to perform hidden API processing
were being created but they were not currently being used which is
good because they fail.

However, adding the fragment to the platform-bootclasspath will cause
those rules to be used as the platform_bootclasspath module performs a
consistency check on the hidden API flags generated by each of the
fragments to ensure that they are consistent with those it generates
itself.

The dynamic bootclasspath work will need to add fragments to the
platform_bootclasspath and without this change that would be blocked
until all fragments had been switched to generating hidden API flags
properly.

This change adds a new fragments property to the bootclasspath and
disables hidden API processing for everything other than ART and tests
if the fragments property or stub libs is empty.

Bug: 179354495
Test: - Before making this change.
      - Add com.android.os.statds-bootclasspath-fragment to platform-bootclasspath.fragments
      m out/soong/hiddenapi/hiddenapi-flags.csv
      - hidden API processing fails in statsd as it cannot find java.lang.Object.
      - After making this change, run the above command again and it should pass.

Change-Id: Ifbb362f8fcfb2c06595fbd5ae39421b536e329ef