Base: Factor out preference-in-storage check to overridable method.

taken from
https://github.com/CyanogenMod/android_frameworks_base/commit/e262881de4fd813bf876ff0b3f2a476d30a1b74d

need by
https://gerrit.omnirom.org/#/c/5759/

Change-Id: I64966e15e0e92a2058809ead2c694a9badae881b

Conflicts:
	core/java/android/preference/Preference.java