Unify "app_parts" path obtaining in MmsProvider

MmsProvider uses different ways to get "app_parts" directory path in
generating part "_data" path and in checking "_data" path prefix. System
may have a bug of not returning consistent path using different calls
with Context. This fix makes sure we use exactly the same API to get
"app_parts" directory path.

b/18838864

Change-Id: I5da55763c337ca310fe3eaf2f940ed6be1bea125
1 file changed