Poll UID stats during iface changes, enforce idx.

When ifaces change, poll UID stats without persisting, since they
depend on knowing active iface to store correctly.  Log dropped UID
stats when iface is unknown.  Switch to using flags when calling
performPoll().

Enforce that "idx" values are consistent from xt_qtaguid.  Transition
to using Log.wtf() for important checks, mostly around file I/O,
kernel stats parsing, and kernel module control.  Increase stats
persist threshold to 2MB to reduce churn.

Bug: 5269476, 5270106
Change-Id: I721215bfb65127f95775c71cf135e907cd567e92