ART: Add generic system-weak holder infrastructure

Add an "interface" for a generic system-weak holder that is
integrated with the well-known instances in Runtime. Add a
simple implementation handling synchronization.

Add a test.

Bug: 31385027
Test: m test-art-host-gtest-system_weak_test
Test: m ART_USE_READ_BARRIER=true test-art-host-gtest-system_weak_test
Test: m ART_DEFAULT_GC_TYPE=SS test-art-host-gtest-system_weak_test
Test: m ART_DEFAULT_GC_TYPE=GSS test-art-host-gtest-system_weak_test
Change-Id: I1100e2cbd9ee57860993d0039de73d197681c542