libsnapshot: Only EnsureImageManager when it is needed.

In functions / code paths where images_ is not used,
don't call EnsureImageManager.

Test: libsnapshot_test
Test: sideload on retrofit
Bug: 140749209
Change-Id: I36635e43b8c2e1c22524f8f4685a216d74c71ad7
1 file changed