NBLog: Make separate readers for dumping logs and histograms

Prior to this commit, the dumping of log contents and histograms from
PerformanceAnalysis were in conflict because histograms obtained data
from the merger's buffer, and the merger used the same readers as the
log dumps. One read would consume the whole buffer, which would cause
either a lossy dump or an lossy merge. Now, both operations can exist
without loss of information from sharing the same reader.

The step of merging contents of each writer's local buffer into a bigger
buffer has effectively been removed. PerformanceAnalysis now directly
reads from each writer's buffer instead of reading from the merger's
bigger buffer.

Test: dumpsys media.log -r
Bug: 68148948
Change-Id: I6d8ea6a8f6a43555183a6d8f17af567506a102f1