Add new system property to override HSUM boot strategy

The new system property allows to override HSUM boot strategy, primarily
targeting testing use case.

Bug: 424267322
Flag: EXEMPT new code path only triggered during test
Test: adb shell setprop persist.user.hsum_boot_strategy 0

Change-Id: I027916b3a787a939a19a96d4d1f852b51ee1e123