Revert "wakelock: use single threaded workqueueus"

This reverts commit 7cf296f8064db7991c4cffb13e07de1b5a981d4a.

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It's still unclear exactly why unbound workqueues increase the Android
OS load measurably (when phone is idle with screen off, which makes
sense as that is when wakelocks use the workqueue), probably it's
because of more tasks spawned.

Change-Id: Ib99ca0ccf727c2419289338aa4916bf90b6a8ef5