Fix getOverrideMinSize to not go beyond min bounds

Fixed getOverrideMinSize in PipSizeSpecHandler to set
a lower bound on the possible overridden min sizes.

Also adjusting unit tests to reflect the new purpose of
getOverrideMinEdgeSize()

Bug: 198643358
Test: atest PinnedStackTests#testEnterPipWithTinyMinimalSize
Change-Id: Ie6c6f3bdbb939156cd0cbd1fb76e680592a9b426