Separate Android Encryption Password from Lockscreen Password

This permits the use of a different (hopefully longer and more complex!) encryption
password, while using a simpler (and easier to type) lockscreen password.

This is similar to what we had in 4.3 minus the option
https://github.com/ParanoidAndroid/android_frameworks_base/commit/5940fc0049b31c8cb9a3e8c377886fcc7232990d

Change-Id: Ia35fdcc769eee8fd7891678a532dafa0f4b3ba04