Fix color stop interpolation when positioned stop has no gap before it

[nduaarte]

Summary:

Fixes #57345

ColorStopUtils.getFixedColorStops() produced incorrect positions when a
color stop with an explicit position was immediately followed by a color
stop without a position (i.e. zero unpositioned stops between them).

The root cause is that lastDefinedIndex was only advanced when
interpolation actually occurred (unpositionedStops > 0). When a positioned
stop was encountered with no unpositioned stops before it, lastDefinedIndex
failed to advance, causing the next interpolation to use a stale start
point and silently overwrite the explicitly declared position of the
skipped stop.

The fix adds an else if branch that advances lastDefinedIndex whenever
the current stop has a defined position, even when no interpolation is
needed at that step.

Changelog:

[ANDROID] [FIXED] - Fix incorrect color stop positions in gradients when a positioned stop is immediately followed by an unpositioned stop

Test Plan:

Added testColorStopsWithPositionedStopAdjacentToUnpositionedStop to
ColorStopTest.kt, covering the exact case from the issue:

Input: [red 0%, green 20%, blue (no position), yellow 80%, purple 100%]
Expected: [0.0, 0.2, 0.5, 0.8, 1.0]

Before the fix this produced [0.0, 0.267, 0.533, 0.8, 1.0].

Verified the new test passes via CI (no local Android environment available).

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