Rename PictureInPictureUiState#isEnteringPip

Renamed to PictureInPictureUiState#isTransitioningToPip per API review
feedback.

Flag: ENABLE_PIP_UI_STATE_CALLBACK_ON_ENTERING
Bug: 326097236
Test: atest CtsWindowManagerDeviceActivity:PinnedStackTests
Change-Id: I5dae2750fbf17468473dd7c685e5213561e18318
4 files changed