Settings: trampoline persist.logd.logpersistd to logd.logpersistd

- Deal with trampoline, where we read logd.logpersistd and
  logd.logpersistd.buffer, write to persist.logd.logpersistd and
  persist.logd.logpersistd.buffer and logd.logpersistd*
- Deal with an as designed requirement that empty values do not
  automatically trampoline, to properly communicate state.
- check logd.logpersistd.enable boolean for support

Bug: 28936216
Bug: 29831823
Change-Id: Ib460ca90738ffdee17084349c1035701301c52bc