Allow ProcessUpdateState to be paused.

Change callback type of ProcessUpdateState to bool() so that, when callback
returns false, immediately returns Merging.

Test: libsnapshot_test
Bug: 147696014

Change-Id: I9dcb8e1658b95216c0f1991e2e4c1ea2e7e7b2e5
Merged-In: I9dcb8e1658b95216c0f1991e2e4c1ea2e7e7b2e5
2 files changed