fix(android-sample): let a guest cart take the shop market

[kieran-osgood-shopify]

What

The Android sample gives a guest cart a hardcoded Canadian buyer identity. This removes it, so
a guest cart takes the market of the shop, like the Swift and React Native samples already do.

- buyerIdentity = Optional.present(CartBuyerIdentityInput(countryCode = Optional.present(CountryCode.CA)))
+ buyerIdentity = Optional.Absent

Why

A country on the buyer identity picks the market. The market decides the currency, the address
form, and the labels on its fields. So the Android sample rendered a Canadian checkout, while
the shared E2E fixture types a United States address.

That is fault B4 in e2e/FLAKES.md: checkout-guest failed on Kotlin Android at
Scrolling DOWN until "Country/Region" is visible, and passed on React Native Android.

Evidence from the two BrowserStack Maestro logs:

Target	Country/Region node	Result
Kotlin Android	Bounds(x=34, y=336, width=1010, height=0), unchanged over 6 swipes	fail after 127s
React Native Android	Bounds(x=34, y=1202, width=1010, height=134)	pass

React Native also skipped the country picker step, because United States was already
selected. The Kotlin failure screenshot shows a total in CAD.

The three samples now agree:

Sample	Guest cart buyer identity
Swift	Storefront.CartInput(lines: lines)
React Native	{}
Kotlin	Optional.Absent

Scope

This changes the sample app only. It does not touch the published com.shopify:checkout-kit
library, so it has no consumer impact.

The demo buyer identity and the signed-in customer paths are unchanged. Only the guest path
loses the country.

Test

Written first, and confirmed red before the fix:

@Test
fun `a guest supplies no buyer identity`() {
    val input = cartInput(demoBuyerIdentityEnabled = false, customerAccessToken = null)

    assertThat(input.buyerIdentity.getOrNull()).isNull()
}

Check	Result
dev android test	BUILD SUCCESSFUL, 36 tests
dev android lint	BUILD SUCCESSFUL

🤖 Generated with Claude Code

[kieran-osgood-shopify]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• fix(android-sample): let a guest cart take the shop market #570 👈 (View in Graphite)
• feat(e2e): add the customer account checkout test on all four targets #562 : 1 other dependent PR (#577 )
• feat(e2e): add the guest checkout order test on all four targets #561
• feat(e2e): add the hardcoded buyer identity order test on all four targets #560
• feat(e2e): add the present-and-close checkout test on all four targets #559
• feat(e2e): add the cart-from-control-link test on all four targets #558
• Replace the E2E suite cross product with Maestro tags in CI #557
• feat(e2e): select tests by Maestro tag instead of scenario flags #556
• feat(e2e): add an E2EController to each sample app #555
• Add the E2E control link parser to all four sample apps #554
• Add sample app test seams and run them in CI #553
• main

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