Remove FlickerPropertyInitializer helper class.

Remove the unnecessary FlickerPropertyInitializer helper class since
BubbleFlickerTestBase already contains all required flicker test
properties. The FlickerPropertyInitializer was creating duplicate
instances of instrumentation, uiDevice, wmHelper, tapl, and testApp
fields that were redundant with the parent test base class.

Consolidate all flicker properties directly into a named companion
object called FlickerProperties within BubbleFlickerTestBase for better
readability and organization. This eliminates the inheritance chain
while maintaining the same functionality. Test classes can still declare
their own testApp in their companion object and override the
BubbleFlickerTestBase#testApp property when needed, such as for tests
requiring specialized app helpers like ImeShownOnAppStartHelper or
ScrollToFinishHelper.

This refactoring simplifies the class hierarchy, reduces code
duplication, and makes the test structure more straightforward without
changing the existing test behavior or capabilities.

Bug: 387193964
Flag: EXEMPT TEST_ONLY
Test: atest WMShellExplicitFlickerTestsBubbles
Change-Id: I57d1c2765646fc6f8c90b68cd0939b00ab846f40