sensorhandler: add accessor for entities singleton

Add an accessor to grab the entities singleton as a step towards
building it on first access from json.

Tested: This change has not been tested beyond build verification.
Signed-off-by: Patrick Venture <venture@google.com>
Change-Id: I2cf1fbd7ad10b4e1b63af347c824ad37bc8a56c9
1 file changed