Add another executor to setSchema() for running framework migrations.

The advice to add a second executor is based on API council guidance,
who do not recommend reusing the callback executor.

The executor is not yet used but will be used once schema migration is
implemented in framework.

Bug: 183177268
Test: Builds
Change-Id: Ic6735e696bc868884adffddd1433a198a005cad6
4 files changed