Rename variables in transformUnderWeaverSecret()

Rename the 'secret' parameter of transformUnderWeaverSecret() to
'weaverSecret', and rename the 'weaverSecret' local variable to
'hashedWeaverSecret'.  This improves consistency, since elsewhere in the
code 'weaverSecret' means the unhashed secret.  No functional change.

Test: atest FrameworksServicesTests:com.android.server.locksettings
Bug: 320392352
Bug: 395976735
Flag: EXEMPT refactor
Change-Id: I734982248e09dc29ff5e561e7da0411ae6bd6868