Skip last checkout screen when there's only one payment option available

[atavism]

Resolves https://github.com/getlantern/engineering/issues/1366

[atavism]

Looks like the TestProxying test is failing..

    android_test.go:179:
        	Error Trace:	/Users/atavism/go/src/github.com/getlantern/lantern-client/internalsdk/android_test.go:179
        	            				/Users/atavism/go/src/github.com/getlantern/lantern-client/internalsdk/android_test.go:96
        	Error:      	Received unexpected error:
        	            	failed to WebSocket dial: failed to send handshake request: Get "https://relay.lantern.io/allocate": context deadline exceeded
        	Test:       	TestProxying
        	Messages:   	should allocate relay

[atavism]

I'm not sure what the status of replay.lantern.io is. But since messaging is disabled, I'm just gonna disable the tests as well.

[jigar-f]

@atavism Just one comment

[atavism]

Thanks for the review, @jigar-f! Just made the changes you requested. Does it look good to you now?

[jigar-f]

Looks good @atavism, Merging.