Switch between sats and BTC now saves in Shared Preferences

Now when the user switches to sats view, the change is remembered even if they close the app or if they browse through "send", "receive", or the post-mix wallet. I used the Shared Preferences tool to create a new field in the already existing Samourai Shared Preferences file. The field is called "IS_SAT" and changes itself from true or false depending on the user's choice.

Second commit: I changed the code to use the PrefsUtil.java file and save it on the backup encrypted file with PayloadUtil.java