Change API to return KeyboardLayout[] to reduce IPC calls

Instead of using String[] to provide available layouts and
Settings calling getKeyboardLayout(desc) on each string, return
all KeyboardLayouts as an array to reduce IPC calls.

Also allowing null ime subtype. This can happen for certain IMEs
that dont have a valid subtype.

Test: manual
Bug: 259530132
Change-Id: I76e5c0b1b98062e917fc8a083c2db16f9b130a58