[AppsFilter] read sysprop only when it's changed

Bisection shows ag/19669172 likely caused a small regression on the
health metrics possibly due to the overhead of parsing.

Given that system properties don't change much, optimize the reading of
the debug property to be called when any system property is changed.

BUG: 241751330
Test: atest AppEnumerationTests
Change-Id: Ib1c71db0e668b10ddb92fd842fa3a18dd2cf323c