Mitigate LSKF leaks through the LockscreenCredential in KeyguardManager

This CL adds try-with-resources around LockscreenCredential objects,
in order to call `close()` when the resource is done being used.
This also clears the LSKF `byte[]` representation in the LockscreenCredential.

This CL applies the above changes to the KeyguardManager.

Bug: 320392352
Test: atest KeyguardManagerTest
Change-Id: Idf1d8545a7bc94a257e939fc41184ea9442694f3