Bug fix: GestureLauncherService is disabled incorrectly

The enable condition only includes camera sysprops/res configs however
the class handles both camera gesture and emergency gesture. When camera
flags are all turned off, the entirely class is turned off by mistake.

This change adds a res config so OEMs can turn on/off emergency gesture
feature individually, instead of bundling camera/emergency together.

Fix: 175236091
Test: atest GestureLauncherServiceTest
Change-Id: I774edb646b96c69549ee883143d76500af32b298