Acquire service lock in AppTaskImpl.moveToFront()

Oversight from previous refactor in O release done in ag/2987716

Change-Id: I2e2d6e55251eb319ddee0db97eedc1c574736ead
Merged-In: I2e2d6e55251eb319ddee0db97eedc1c574736ead
Fixes: 111336376
Test: builds
(cherry picked from commit bc4d0326a04c19500f44283fbfab581257d3c75e)
1 file changed