SF: Fix a scheduler related mock leak

After a recent change to inject mocks for use in the scheduler, it turns
out the mocks were being leaked, and the test was complaining about
such. However `atest libsurfaceflinger_unittest` hides those messages.

If you run the test directly, it complains "ERROR: 276 leaked mock
objects found at program exit." with a bunch more information about
which ones, and which tests were the source.

Its a one line change to fix the leak in all tests. This patch does so.

Test: libsurfaceflinger_unittest
Bug: None
Change-Id: Ifb4969c16dcf17a7c55777a3a3e3896273cab68f