Export monolithic hidden API files from platform_bootclasspath

Makes the monolithic hidden API files accessible from the
platform_bootclasspath so they can be output to the dist build target
and used by other modules, e.g. by doing something like this:
  java_resources: [
    ":platform-bootclasspath{hiddenapi-flags.csv}",
  ],

It makes the paths relative to the out/soong/hiddenapi directory rather
than the out/soong directory to make them easier to use in the
java_resources property without changing the structure of the APK.
Without that attempting to use them in a java_resources property will
result in them being copied to a hiddenapi/ within the APK instead of
being used at the top level as existing APKs like
CtsHiddenApiBlocklistTestApiTestCases expect.

Bug: 177892522
Test: m nothing
Change-Id: I829412fc7d25411e0c2e0713d0d219a18f4af2ee