Instead of coalescing for 15 seconds after the first change
notification, coalesce until change notifications have been idle for at
least 2 seconds. This avoids long update delays, which is especially
jarring when using notifications on a wearable and the initial
notification didn't yet include the message body.
Also skip coalescence entirely for deletions; update immediately in that
case.
Change-Id: I67bed9a1af7b023020b0fd5429495eb45000e858
a29e6b2215