Restrict the overridden min size for PiP

Per go/cdd-proposal-pip-size, the absolute minimum size when overridden
should be 48dp.

Bug: 174302616
Test: atest PinnedStackTests#testEnterPipWithTinyMinimalSize
Merged-In: I2f0dfd9a79049aaf696fde8703994deaecce63f5
Change-Id: I2f0dfd9a79049aaf696fde8703994deaecce63f5