Only reset mExpectingSynthesizedDown once expansion actually starts

Previously, when expanding the shade via external input, mPanelExpanded would store an intermediate value between NPVC receiving the remote signal and the expansion height actually becoming non-zero. In order to avoid this, mExpectingSynthesizedDown is now not reset when we use it to trigger a fling, but rather when that fling either actually increases the expansion height to non-zero, or is cancelled.

Bug: 365234590
Test: Manually verified via logging that mPanelExpanded value is now only set once (and correctly) on shade expansion
Flag: EXEMPT bugfix
Change-Id: Ibf34b327620956a7f314f733e44d32c64ebfc5e5