startop: Update to toggle iorapd.readahead.enable.

Test: python run_app_with_prefetch.py  -p com.android.settings -a com.android.settings.Settings -r fadvise -i input --debug --simulate
Test: python run_app_with_prefetch.py  -p com.android.settings -a com.android.settings.Settings -r fadvise -i input
Test: pytest run_app_with_prefetch_test.py

Bug: 135286022
Change-Id: I4576ebb66bc41124e419681a296a47591f1f3d09