Fix NullPointerException in DreamManagerService

mCurrentDreamName has one access from an unlocked context, which causes
a race condition between the dream being stopped and started. This
causes a NullPointerExcpetion.

This CL fixes this by moving the mCurrentDreamName read operation back
into a locked block.

Bug: 209911670
Test: m
Change-Id: Icb8e904f067199fbfa62ff90aecd57bf38015546