Modularize logd.

Separates logd body into a static library liblogd and
virtualize LogBuffer::log to be in a new interface class
LogBufferInterface. User could have different implementation.

Bug: 37756450
Test: liblog-unit-tests, logd-unit-tests and logcat-unit-tests
with (b/37791296).
Change-Id: I1504ff0e992744001b5a2e9abd45692d1318a152