Change key-source separator in TimeInStateHelper

The injectOption (tradefed functions) already used ":" and "=" as
separators, we need to change the separator to another character ("@").

This will also add a checking whether the source file exists or not
during setUp().

Test: TimeInStateHelperTest and TimeInStateListenerTest
Bug: 209531420
Change-Id: I4731590df75a398a664a76da1769d80ac55f5f29