Improve ANR dropbox reports

Work towards better diagnosing b/29501073. Adds logic to ensure that the dropbox
entry generated for ANRs fits at least some part of the logcat before the MAX_DROPBOX_SIZE
mark. Also reduces the MAX_DROPBOX_SIZE to be better match size restrictions.

Bug: 29501073
Change-Id: Ice5599582cbb536b7d81aa0c0340ff753ca86ebf