Move hidden API index file rule to platform_bootclasspath

This change moves the monolithic hidden API index file creation rule
from the hiddenAPIIndexSingleton into the platform_bootclasspath. It
also moves the corresponding test from java/hiddenapi_singleton_test.go
to java/platform_bootclasspath_test.go.

Bug: 179354495
Test: verified that the out/soong/hiddenapi/... files are unchanged
      by this change
Change-Id: Ia295d0f7ae9b51ea816f16921aa42339ed91704e