Move handling of prebuilt hiddenapi-index.csv to hiddenapi singleton

This change moves the handling of the prebuilt hiddenapi-index.csv file
from the hiddenAPIIndexSingleton and removes that as it no longer
needed.

Bug: 179354495
Test: verified that the out/soong/hiddenapi/... files are unchanged
      by this change
Change-Id: Iab1b39a57f28caa855c48538aa2230795f2c3271