Fix lib name resolution if extension is substring.

If a library happens to contain the extension as a substring in its
name, libNameFromFilePath will return a truncated library name. Change
this calculation to remove the last instance of the extension substring
instead.

Bug: 147140513
Test: Modified rust tests pass.
Change-Id: I0ed91e5f571ed5c4040ee15956a1598846aee43a