libprefetch: check `ro.prefetch_boot.enabled` internally

Checking this value on the prefetch internally prevents
it from starting unless you explicitly enable the feature.

Bug: 380766679
Test: Builds
Test: Build emulator image with `ro.prefetch_boot.enabled 0`.
observe prefetch record exiting even when other condition meets.

Change-Id: Iadce8bd77a6b5299f33efdf5e195e715798f5104