Add File.separator to end of internalPath

When compiling theme resources, packages that start with the same
internal path end up being processed by AAPT which can cause
conflicts and not allow the theme to be installed.  An example of
this is a theme that overlays both com.google.android.gm and
com.google.android.gms.  When the overlay for com.google.android.gm
is being processed, resources from com.google.android.gms are also
included.  This is easily resolved by appending a file separator
to the end of internalPath.

Change-Id: I98f7e0fd65c54bdd854df1539e815ad76cfa44ae
(cherry picked from commit e56a4b2a)