docs: Updates to API Overview for N Preview 2
docs: Updates to API Overview for N Preview 2

Adding new material based on PM feedback. See first comment for doc
stage location.

Change-Id: Iab58bd10804bf1e088752eb385b352d91a0e5d2c

xorware/android_frameworks_base,xorware/android_frameworks_base,xorware/android_frameworks_base,xorware/android_frameworks_base,xorware/android_frameworks_base,xorware/android_frameworks_base