check-flagged-apis: api-versions.xml: correctly parse nested class ctor

The constructor of a nested class is represented as follows in
api-versions.xml:

  <class name="android/Clazz$Foo$Bar" since="1">
    <method name="&lt;init>()V"/>
  </class>

The nested dollar signs are not replaced by forward slashes before the
parsing logic uses `split("/")` to find the name of the inner-most
class, incorrectly resulting in `Class$Foo$Bar` instead of `Bar`. Fix
this by immediately replacing dollar signs with forward slashes after
extracting the package and class.

Also clean up the following call of `Symbol.create`.

Bug: 334870672
Test: atest --host check-flagged-apis-test
Change-Id: I8c0619faae90ded7eb14dcc20ecb94a086a1c764