Add unit tests for odrefresh - step 2

- Define MockableExecUtils, which is a wrapper class to make exec_utils
mockable.
- Add another constructor to the OnDeviceRefresh class to support
injections.
- Update OdrConfig to support overriding the staging directory.
- Add a unit test to verify that odrefresh sets compiler filter based on
"dalvik.vm.systemservercompilerfilter".

Bug: 196188549
Test: atest art_standalone_odrefresh_tests
Test: atest odsign_e2e_tests
Change-Id: Ia7b4347bba6873878490a513b8a5ff6642122d71
7 files changed