Remove long unused (and non-user-modified) implicit ZenRules

We now track the last time each ZenRule was activated. On ZenModeConfig load (e.g. reboot, user switch) if 30 days have passed since the last activation of an implicit rule, it is deleted. For rules created before we started tracking activation, we also set lastActivation=now -- in essence, so that we wait 30 days before deciding whether they are actually used or not.

As a side-change, rename ZenRule.canBeUpdatedByApp() to ZenRule.isUserModified(), inverting is logic. The latter is the condition that almost all callsites are interested in.

Bug: 394087495
Test: atest ZenModeConfigTest ZenModeHelperTest
Flag: android.app.modes_cleanup_implicit
Change-Id: If9f7916e59b47871f14bd4b1b4efaf6cd2d41af7