Guard MediaDeviceManager.Entry against successive stop calls

It's non obvious from the code that an entry will be stopped
only once. The start() protection is added for simmetry.

Bug: 232812007
Test: Manual behavioral test.
Test: atest MediaDeviceManagerTest MediaDataCombineLatestTest
Change-Id: Icb4573d30790b4055b67eff57c495ceb6c3ae61b