Enable Freezer by default

The freezer init code will check if the Linux kernel is new enough to
support cgroupfs v2. If it does, freezer will be enabled by default.
Otherwise, freezer will be disabled.

Some tests were not correctly designed. The logic was wrong. This CL
also fixes those broken tests.

Meanwhile, CachedAppOptimizerTest should ignore freezer related tests if
freezer is not supported. Furthermore, the mockingservicestests isn't
running within system_server. So the current test always reports freezer
is not supported. This should be fixed separately.

Bug: 195589090
Bug: 195690032
Test: adb shell dumpsys activity settings | grep use_freezer=true
Test: atest CachedAppOptimizerTest
Change-Id: If994e452c9771b94130aa8b91240dcf924435710