Split cache clearing into two phases.

Use newly defined "V2_DEFY_QUOTA" flag to split cache clearing into
two phases: clearing data for apps above their quotas, and then
pushing deeper by clearing data for apps below their quotas.

Add placeholder comments for other data types that we're planning to
add shortly.  Route all clearing behavior through this new method,
which remains guarded behind a flag for now.

Test: builds, boots
Bug: 34692014
Change-Id: I678d7b4e2bf6c837dd8a9adbc36a53015907f75f