libaudiohal@aidl: Use system property to detect IHalAdapterVendorExtension

Use of the device manifest for this purpose was discouraged after
a discussion. The alternative is to use a system property which
needs to be set by vendors together with putting an implementation
of IHalAdapterVendorExtension to the `system_ext` partition.

Remove the vintf fragment to avoid confusing vintf assembly and test
processes.

Bug: 323989070
Test: atest audiorouting_test
Change-Id: If00ec0653590cf2f078d94bc57dfce47d8e98e01