Add Extensions API

Please see the commit message of change Ia9b99015eec7a48bbf969cbe503862271f09adca
for motivation.

Bug: 118604960
Bug: 118606929
Test: NeuralNetworksTest_static
Change-Id: I2703b963f040a846889554888ddd984eac6b6c08
Merged-In: I2703b963f040a846889554888ddd984eac6b6c08
(cherry picked from commit 2543307e0a5caa66abc67cea0a5fe8244e442712)
24 files changed