logd: always wake 'wrapped' readers on prune

See the comment in the code for more details.  'wrapped' readers are
uncommon and error prone, and this change makes them more reliable.
Its side effect is that wrapped readers will wake more often, but
they'll still be batched to a large degree.

Bug: 163617910
Test: logging unit tests
Test: logcat --wrap does the right thing
Change-Id: I4b6f8331ff7854787c97f821b2a5bf8d7da321c6