Use InstrumentedFragment for EncryptionInterstitial

Since EncryptionInterstitial now uses buttons and not preference list
items, extend InstrumentedFragment rather than
SettingsPreferenceFragment for less overhead.

Test: Run EncryptionInterstitialTest via `am instrument`
Change-Id: Idb56b467ae03a1aff680dbc25d2889dad77f391d