Move reading sysprops out of the Flattener.

Flattener cannot read sysprops directly as otherwise FlattenerTest may
begin to use device-specific toggles in a test suite that is meant to be
device-agnostic

Bug: 196210513
Test: libcompositionengine_test
Change-Id: I65a4a18f7e609d4a0f70a4a49fae5cd1f85d12a2