Add PipBoundsState to manage current pip bounds

This change adds a basic state class and migrates
mLastReportedBounds from PipTaskOrg into it. It'll
eventually become the holder of all bounds-related state
such as movements bounds, reentry bounds/fraction and
aspect ratio.

Bug: 169373982
Test: atest com.android.wm.shell.pip
Change-Id: I43c530f4f3ab192756964022417f39f3085e593e