By default do not spawn a thread to perform CPU execution

This simply changes the debug.nn.syncexec-cpu default
from 0 to 1.

Bug: 123757760

Test: NeuralNetworksTest_static

Test: NeuralNetworksTest_static --gtest_filter=GeneratedTests.mobilenet_224_gender_basic_fixed
      with debug.nn.vlog set to all; try all three values for debug.nn.syncexec-cpu (unset, 0, 1)
      and verify by looking at the logcat that compilation and execution
      ran on the same thread for unset and 1, and different threads for 0.

Change-Id: I9b230e545163cb53b12cd6628d54f6b8596b08e0
Merged-In: I9b230e545163cb53b12cd6628d54f6b8596b08e0
(cherry picked from commit b7972553)