Use isGestureNav signal from TaskbarActivityContext for getTaskbarPhoneDimensions

Somehow deviceProfile.isGestureMode doesn't have the right signal for tests.

We need to look into why dp doesn't have the right signal for isGestureMode in tests. Temporary fix here to unblock pushing the flag to staging

Bug: 359196354
Test: https://android-build.corp.google.com/builds/abtd/run/L05600030005823769
Flag: com.android.wm.shell.enable_taskbar_on_phones
Change-Id: Ie6451812cf4b0c3f034cabdfd4cec726322af0bc