Avoid mentioning complex deprecated API in API docs

In getBounds() API doc, the code snippet to get the window size
excluding system decorations refers it as the legacy value of getSize()
API. As the getSize() behavior becomes more complex, to explicitly
describe the behavior instead of referring to it can make things clear.

Bug: 151861875
Test: N/A
Change-Id: If59ef701b833727885ea3baa16b0f6804c05cba8