Ravenwood support for `SystemProperties`.

One of our final missing pieces of foundational functionality is
the SystemProperties key/value store.  Over the years, this key/value
store has been (ab)used to configure very obscure parts of the OS.

As tempting as it might be to simply let code rely on default return
values when a key is undefined, we'd like to ensure that code owners
carefully confirm any assumed behaviors.  To accomplish this, we
default to blocking both read/write access to keys until their use
has been explicitly audited.

Based on our guiding principles, as code owners support their APIs
under Ravenwood, they're expected to bring along all relevant tests,
which will uncover SystemProperties usage that needs triage, reducing
the risk of downstream clients uncovering that usage.

Tests can explicitly allow read/write access to specific properties
via their RavenwoodRule.Builder definition, which is also how we
ensure that all values are consistently reset between tests.

Bug: 319647875
Test: atest FrameworksCoreSystemPropertiesTestsRavenwood
Test: atest FrameworksCoreTestsRavenwood CtsOsTestCasesRavenwood
Change-Id: I6510e06c33ee8b2bf31b58f35faa07127ecd16b7