Refactor bubble helper for clarity and KDoc.

This change refactors the BubbleFlickerTestHelper to improve code
clarity, adopt more idiomatic Kotlin, and add KDoc.

The dumpBubbles extension function has been renamed to describeAll to
better reflect its purpose. Its implementation is updated from a manual
StringBuilder loop to a more concise joinToString call.

Additionally, KDoc is added to the switchBubble method and the renamed
describeAll function to improve documentation. The code formatting
within switchBubble is also improved for better readability.

Bug: 387193964
Flag: EXEMPT TEST_ONLY
Test: atest WMShellExplicitFlickerTestsBubbles:SwitchBetweenBubblesTest
Change-Id: I0b4d4263faeedd5d585762cd2974f3e9dfefd6c0