Rename alpha and refactor the way alpha is set in PageIndicator

Here we rename DOT_INACTIVE_ALPHA and DOT_ACTIVE_ALPHA to reflect the new code, and we also use mPaginationPaint.setAlpha a bit differently

Bug: 247561880
Test: Manual
Change-Id: I5ee25ee3fb36715d54f9ed86c0d59448829d7260
1 file changed