TextDumpsysSection memory optimization

TextDumpsysSection used to inherent WorkerThreadSection, which
allocates 2x more memory than the dumped content. This change saves
the extra allocation by writing dumpsys content directly to FdBuffer.

Bug: 150311553
Test: Manually run "incident -p EXPLICIT 4000"
Change-Id: I9c0c0db75c8595822ee0711040e8865dd69378b6