Add some guidance documentation to the emulator

Because fingerprint HAL is vendor-specific due to the
Trusted Execution Environment (TEE) requirements, source
is not provided in AOSP. As such, this example is a
reasonable place to document some of the requirements
and expectations.

Fixes bug 23045367

Change-Id: Iacf14d6d32b468464215320b411ef7ff1a387ff0
1 file changed