logd: single std::mutex for locking log buffers and tracking readers

There are only three places where the log buffer lock is not already
held when the reader lock is taken:
1) In LogReader, when a new reader connects
2) In LogReader, when a misbehaving reader disconnects
3) LogReaderThread::ThreadFunction()

1) and 2) happen sufficiently rarely that there's no impact if they
additionally held a global lock.
3) is refactored in this CL.  Previously, it would do the below in a loop
  1) Lock the reader lock then wait on a condition variable
  2) Unlock the reader lock
  3) Lock the log buffer lock in LogBuffer::FlushTo()
  4) In each iteration in the LogBuffer::FlushTo() loop
    1) Lock then unlock the reader lock in FilterSecondPass()
    2) Unlock the log buffer lock to send the message, then re-lock it
  5) Unlock the log buffer lock when leaving LogBuffer::FlushTo()
If these locks are collapsed into a single lock, then this simplifies to:
  1) Lock the single lock then wait on a condition variable
  2) In each iteration in the LogBuffer::FlushTo() loop
    1) Unlock the single lock to send the message, then re-lock it

Collapsing both these locks into a single lock simplifes the code and
removes the overhead of acquiring the second lock, in the majority of
use cases where the first lock is already held.

Secondly, this lock will be a plain std::mutex instead of a RwLock.
RwLock's are appropriate when there is a substantial imbalance between
readers and writers and high contention, neither are true for logd.

Bug: 169736426
Test: logging unit tests
Change-Id: Ia511506f2d0935a5321c1b2f65569066f91ecb06