Cherry-pick #19667 to 7.0

[micahgodbolt]

Cherry-pick #19667 to 7.0
Fix Dropdown to only add ARIA live region when focused

[codesandbox-ci]

This pull request is automatically built and testable in CodeSandbox.

To see build info of the built libraries, click here or the icon next to each commit SHA.

Latest deployment of this branch, based on commit 4a01488:

Sandbox	Source
Fluent UI Button	Configuration
microsoft/fluentui	Configuration

[size-auditor]

Asset size changes

Project	Bundle	Baseline Size	New Size	Difference
office-ui-fabric-react	office-ui-fabric-react-Dropdown	230.237 kB	230.268 kB	31 bytes

 Over Tolerance (1024 B)  Over Baseline  Below Baseline  New   Removed 1 kB = 1000 B

Baseline commit: 0fc7218de6115b33c0ba00c0cebeb377c56f945a (build)

[fabricteam]

Perf Analysis

No significant results to display.

All results

Scenario	Render type	7.0 Ticks	PR Ticks	Iterations	Status
BaseButton	mount	799	784	5000
Breadcrumb	mount	37513	38081	5000
Checkbox	mount	1362	1351	5000
CheckboxBase	mount	1128	1100	5000
ChoiceGroup	mount	4304	4211	5000
ComboBox	mount	769	772	1000
CommandBar	mount	6860	6862	1000
ContextualMenu	mount	11847	11924	1000
DefaultButton	mount	964	961	5000
DetailsRow	mount	3248	3224	5000
DetailsRowFast	mount	3274	3296	5000
DetailsRowNoStyles	mount	3069	3093	5000
Dialog	mount	1530	1529	1000
DocumentCardTitle	mount	1589	1583	1000
Dropdown	mount	2150	2129	5000
FocusTrapZone	mount	1556	1495	5000
FocusZone	mount	1588	1545	5000
IconButton	mount	1491	1527	5000
Label	mount	286	285	5000
Layer	mount	1693	1715	5000
Link	mount	361	374	5000
MenuButton	mount	1241	1247	5000
MessageBar	mount	1806	1807	5000
Nav	mount	2874	2846	1000
OverflowSet	mount	1241	1323	5000
Panel	mount	1323	1368	1000
Persona	mount	884	831	1000
Pivot	mount	1255	1276	1000
PrimaryButton	mount	1133	1185	5000
Rating	mount	6739	6818	5000
SearchBox	mount	1135	1103	5000
Shimmer	mount	2245	2312	5000
Slider	mount	1309	1344	5000
SpinButton	mount	4687	4472	5000
Spinner	mount	372	388	5000
SplitButton	mount	2815	2710	5000
Stack	mount	432	412	5000
StackWithIntrinsicChildren	mount	1485	1456	5000
StackWithTextChildren	mount	4229	4091	5000
SwatchColorPicker	mount	9289	9163	5000
TagPicker	mount	2549	2458	5000
TeachingBubble	mount	44483	44247	5000
Text	mount	359	353	5000
TextField	mount	1269	1199	5000
Toggle	mount	723	731	5000
button	mount	106	105	5000