Prevents posting multiple pulse requests at the same time

When docked, DozeDockHandler will try to request pulse almost every time
doze machine transits to DOZE or DOZE_AOD state. Guards pulse requests
with flag to prevent requesting multiple pulses at the same time.

Bug: 135226053
Bug: 135396479
Test: Manual test and the symptom gone.
Change-Id: I20d7def12c25b2772ef2c1ebefba4b4ac4d313d6