Make bootclasspath_fragment hidden API package checks exhaustive

Previously, the bootclasspath_fragment's hidden_api.split_packages and
hidden_api.package_prefixes properties did not specify an exhaustive
set of packages that were provided by the fragment. They excluded
packages which were either not split or which could not be used as a
package prefix because it would match sub-packages provided by other
bootclasspath modules.

This change adds the hidden_api.single_packages list to specify those
additional packages and then uses that information to verify that any
bootclasspath_fragment that specifies at least one of split_packages,
single_packages or package_prefixes properties only contains classes
from a package that matches one of those properties. That will
prevent a module from accidentally including unexpected classes, such
as might happen when statically including a common utility library.

It also adds coverage specific versions of the properties as additional
packages are added to the art-bootclasspath-fragment when building
coverage builds.

Bug: 194063708
Test: atest signature_patterns_test
      m out/soong/hiddenapi/hiddenapi-flags.csv
      m EMMA_INSTRUMENT=true EMMA_INSTRUMENT_FRAMEWORK=true out/soong/hiddenapi/hiddenapi-flags.csv
      # Breaks without corresponding change to add android.system to
      # the art-bootclasspath-fragment.
      /usr/bin/pylint --rcfile $ANDROID_BUILD_TOP/tools/repohooks/tools/pylintrc scripts/hiddenapi/signature_patterns*.py
      pyformat -s 4 --force_quote_type single -i scripts/hiddenapi/signature_patterns*.py
Change-Id: Iddf6c59cd4dc8c36dde7943a9840ccef5794b320