Additional multi client scenarios

[fractalwrench]

Adds the following scenarios when multiple clients are initialised:

• Calling notify while offline is only sent to original client
• Unhandled error while offline is sent twice with different API key for each
• Unhandled error while online is sent twice with different API key for each
• Unhandled error while offline is sent twice with different endpoint for each
• Cached sessions with different API keys are only sent to original client

This also includes a minor change to the TestHarnessHooks which is required to flush the sessions on a per-client basis

Relies on #286

[coveralls]

Pull Request Test Coverage Report for Build 1304

• 0 of 0 (NaN%) changed or added relevant lines in 0 files are covered.
• 19 unchanged lines in 3 files lost coverage.
• Overall coverage increased (+0.5%) to 74.052%

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Files with Coverage Reduction	New Missed Lines	%
sdk/src/main/java/com/bugsnag/android/Client.java	3	58.74%
sdk/src/main/java/com/bugsnag/android/DeviceData.java	3	76.69%
sdk/src/main/java/com/bugsnag/android/SessionTracker.java	13	57.89%

Totals
Change from base Build 1301:	0.5%
Covered Lines:	1855
Relevant Lines:	2505

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💛 - Coveralls