Respect reserved event count for postEventOrFree

Continuous sensor data can quickly flood CHRE's event queue when several
sensors are enabled at a max rate and CHRE stalls when issuing an
external request for some reason.

To avoid this, modify postEventOrFree to respect the
kMinReservedHighPriorityEventCount var that reserves space for high
priority events and migrate several methods that (mistakenly) use this
API to the postEvent method to guarantee their event delivery.

Additionally, modify the postEvent method to become postEventOrDie and
only allow system events to be posted here which also requires moving
nanoapp events to the new postLowPriorityEventOrFree function.

Bug: 139448882
Test: Load onto device and play w/ a_sns_test and see if crashes occur
Test: ./load_chre, ./run_tests.sh, ./run_sim.sh
Change-Id: I1e6d0268b96934b805aaa0dd8ee831e05f74f0f7
Merged-In: I1e6d0268b96934b805aaa0dd8ee831e05f74f0f7