This fixes some small niggles:
1. `inferContentType(String)` is documented to take a path, but in the
tests we're passing full URIs.
2. A `String` parameter is usually a path, but also a MIME type or an
extension. In the new methods, the meaning of a `String` parameter
is always clear from the name of the method.
3. `inferContentType(String)` is always passed an extension in
'production' code (which has to be manually prefixed with a dot).
4. `inferContentType(Uri, @Nullable String)` always ignores the Uri if
the String is non-null. IMO this logic is clearer to a reader if it's
just in-lined at the call-site.
These methods are used from the demo apps, so will be part of the stable
API.
PiperOrigin-RevId: 444826053
