Remove Activity Recognition from the Android Code.

Removing AR from the Android code base. We had already severed the
connection to the AR HAL in Android Oreo. This change simply removes all
unused references and uses of the stubs.

Test: Build compiles. Ensure that clients that provide AR through other
channels continue to work.

Change-Id: I79d19c0bd2b80962cdecc8ad88065911fd6311ac