Add a test for sign-in

This requires an account to exist before as we can't restart our lifecycle.
So we don't automatically clear app data after each test, but rather need to delete an existing account manually before each test.

briar-android/build.gradle

@@ -37,7 +37,7 @@ android {
 		buildConfigField "Long", "BuildTimestamp",
 				"${getStdout(['git', 'log', '-n', '1', '--format=%ct'], now)}000L"
 		testInstrumentationRunner 'org.briarproject.briar.android.BriarTestRunner'
-		testInstrumentationRunnerArguments disableAnalytics: 'true', clearPackageData: 'true'
+		testInstrumentationRunnerArguments disableAnalytics: 'true'
 	}
 
 	buildTypes {