Move hidden API metadata file rule to platform_bootclasspath

This change moves the monolithic hidden API index file creation rule
from the hiddenAPISingleton into the platform_bootclasspath.

Bug: 179354495
Test: verified that the out/soong/hiddenapi/... files are unchanged
      by this change
Change-Id: Ib25be3618e30a83dc2929a18062eb58eefdabefb