Add an api call to check if legacy hal is started

This commit adds a public api call for legacy hal to check if it is
successfully started.
This is to be used by callers of legacy hal to make sure not to request
functionality while legacy hal (and hence all interfaces) are not
started.

Bug: 112205428
Test: Manual
Change-Id: I5183d01811e35edb3149cfb3784067c68b40e46d