Move "Enable Boot Aware Startup data" to behind a FeatureFlag.

Previously this was hard-coded to disabled, but that was before
FeatureFlags were made to be boot aware.

Bug: 251502424
Test: Verified that doing this doesn't break my device. Also tested the
Pre-loading feature itself and didn't see any errors in the logs.

Change-Id: I031e1756b985664a2e8d8ad26e16803ead874727