Nativeloader: Restrict anonymous namespace creation

Create the anonymous namespace only from a non-empty library path.

This is a better heuristic than the current "first-come" one. It
allows creating classloaders that are not directly related to an
app's "main" classloader, that is, do not have any library path.
This is the case, for example, for shared libraries, including
preloaded ones.

Longer-term, the anonymous namespace creation should be made
explicit, so that the framework can have full control.

(cherry picked from commit b9df7d93)

Bug: 130623656
Test: m
Test: manual
Merged-In: I8251363b372e365d10ae09d23df93d76388ac7f9
Change-Id: I8251363b372e365d10ae09d23df93d76388ac7f9