Support UNSAFE_DISABLE_HIDDENAPI_FLAGS in platform_bootclasspath

Previously attempting to use m UNSAFE_DISABLE_HIDDENAPI_FLAGS would
cause a build failure.

It is necessary when UNSAFE_DISABLE_HIDDENAPI_FLAGS=true to create
empty files for the monolithic files so that those usages which are not
protected by the build flag, e.g. in dists or CTS tests like
CtsHiddenApiBlocklistTestApiTestCases do not cause ninja failures due
to reliance on a file that does not exist and has no rule to create it.

Bug: 177892522
Test: m nothing
      m UNSAFE_DISABLE_HIDDENAPI_FLAGS=true CtsHiddenApiBlocklistTestApiTestCases
Change-Id: I05d0b3680dbc0b5b7c7c0d792550faf848829647