ConnectivityService: simplify WakeLock management

This patch simplifies wakelock management by acknowledging that only one
acquisition at most is in flight at any time. This allows to remove the
serial number associated with wakelock acquisition and to avoid double
release when regaining a default network.

Example of $ adb shell dumpsys connectivity logs:
  NetTransition WakeLock activity (most recent first):
    03-31 00:15:13.816 - RELEASE (EVENT_CLEAR_NET_TRANSITION_WAKELOCK)
    03-31 00:14:59.216 - ACQUIRE for NetworkAgentInfo [WIFI () - 101]
    03-31 00:09:05.799 - RELEASE (EVENT_EXPIRE_NET_TRANSITION_WAKELOCK)
    03-31 00:08:05.738 - ACQUIRE for NetworkAgentInfo [WIFI () - 100]

Test: watched wakelock section $ adb shell dumpsys connectivity while
      turning on and off Wifi (with no other network).
Bug: 36703718
Change-Id: I899b0816c0e41b3991d9540e9b9a604914ff673a