Optimize memory usage in incidentd

EncodedBuffer is used a lot in incidentd. EncodedBuffer uses malloc
internally to acquire memory. Frequently creating and destroying
EncodedBuffer creates memory fragmentation, leading to high memory
usage after taking an incident report.
Also fixes a few other places with lots of malloc/free operations.

This change:
* Creates a pool of EncodedBuffer in incidentd. The saving is
significant. It reduces EncodedBuffer creation from 3 per section to
3 per report.
* Replaces malloc with mmap inside EncodedBuffer. mmap is guaranteed
to be mem page aligned, so there will be no mem fragmentation after
destroying EncodedBuffer.
* Replaces new with mmap inside TombstoneSection
* Forks a process to execute LogSection, because liblog malloc & free
significant amount of memory

Result:
PSS before taking a report: 1295 KB
PSS after taking a report: 1336 KB

Bug: 150311553
Test: heapprofd
Change-Id: I83bd9c969b751c80b2f42747020799bd85d8aae6