feat: Add mobile endpoint to retrieve UIAutomator page source dump

[pr4bh4sh]

Problem: driver.page_source doesn't return page source when AUT is not in focus.

We had an issue while working on an application flow where the test navigates outside of the AUT's context. The test performs a photo upload operation as part of the test flow. When the active app is not AUT we use driver.execute_script('mobile:uiautomator',*args) to perform the UI interactions.

When a test fails our framework attaches the screenshot and page source of the active screen. But when the active app is not AUT, espresso driver can't return the page source with driver.page_source. To overcome this we can dump the UI dom with UIDevice.

Problem with the current implementation:

1. UI dump returned by this PR's change is not ready to be consumed by the Appium Inspector.
2. I'm unsure whether this is the most suitable approach.

I don't expect this PR change's to be merged, this is just what works for me now. Perhaps @mykola-mokhnach or @KazuCocoa can suggest a more refined solution for this.

[mykola-mokhnach]

Why not to create another mobile command to get the uia page source? I don't think this is a good idea to mix Espresso's and uia2 output under the same endpoint

[mykola-mokhnach]

kotlin allows to return directly from try: https://kotlinlang.org/docs/reference/exceptions.html#try-is-an-expression

[pr4bh4sh]

fixed

[mykola-mokhnach]

This method could be simplified as

override fun handleInternal(params: AppiumParams): String = try {
    val stream = ByteArrayOutputStream()
    InteractionHelper.getUiDevice().dumpWindowHierarchy(stream)
    String(stream.toByteArray())
} catch (e: IOException) {
    throw AppiumException("Could not get page source with UiAutomator", e)
}

[pr4bh4sh]

done