MH2 | Add rough proxy callback postEvents method

Add post events method to HalProxyCallback without fully implementing
the wakeup events handling and pending blocking writes. Also change
mSensorList of the halProxy to mSensors which is a map from sensorHandle
to sensorInfo so that getNumWakeupEvents method of callback can find
sensorinfo info from sensorhandle of event type. Instantiate
halproxycallback vector in HalProxy for each subhal in the ctor as well.
Add very simple test for the postEvents method of callback.

Bug: 136511617
Test: New unit test passes.
Change-Id: I39c861cff286f24992bfcfcaa6bb468b4544b0e0