ANDROID: Incremental fs: make remount log buffer change atomic

Read log buffer can have multiple threads doing any of these
operations simultaneously:
- Polling for changes
- Reading log records
- Adding new log records
- Updating log buffer size, or enabling/disabling it completely

As we don't control the userspace, and it turns out that they
all currently originate from different processes, code needs to
be safe against parallel access to a read buffer and a request
for reallocating it.

This CL add an r/w spinlock to protect the buffer and its size.
Each remount takes the write lock, while everything else takes
a read lock. Remount makes sure it doesn't take too long by
preallocating and precalculating all updates, while other
operations don't care much about their critical section size -
they all can still run together.

Bug: 152633648
Test: manual remount + reading
Signed-off-by: Yurii Zubrytskyi <zyy@google.com>
Signed-off-by: Paul Lawrence <paullawrence@google.com>
Change-Id: I7271b4cb89f1ae2cbee6e5b073758f344c4ba66a