Fix DocuementsUI can start multiple ResolverActivity

Add intent flag FLAG_ACTIVITY_SINGLE_TOP to prevent ResolverActivity
be launched if it is already running at the top of the history stack.

Bug: 137065134
Test: manual
Test: atest DocumentsUIGoogleTests
Change-Id: Ie42f31b4fc6f95f02b53b251426dc9cb21fc4c91