Add accessibility manager enabled check in progressbar

formatStateDescription() in onProgressRefresh() is causing performance
degradation with accessibility off.

Note that stateDescription is set in onInitializeAccessibilityNodeInfo
to ensure that accessibility always get the update-to-date
stateDescription. Otherwise, stateDescription might be stale on first
accessibility focus and the user might get wrong information.

Fix: b/166215147

Test: tested with talkback enabled from the beginning or from the
middle.

Change-Id: I9fb99b479b172d87c8d0fa1b8765dab53445b993