Move NSC classes in platform to packages/NetworkSecurityConfig

The packages/NetworkSecurityConfig directory will contain two children:
 * "platform" with the current NSC implementation.
 * "module" that will contain wrappers that redirect to the
   new NSC implementation in Conscrypt.

The switch between the two different implementation is controlled
by the release_conscrypt_nsc flag.

This pattern will allow us to ramp up existing callers to the
new implementation without changing the package and signature
names of the current classes and method.

Bug: 397646538
Test: build-flag --all-releases set RELEASE_CONSCRYPT_NSC false && m
Flag: build.release_conscrypt_nsc
Change-Id: Iddb2f2761558c79ea3299a561d023d9b7925f829