Translation: use Android's cancel/ok.

Fix #868.

src/main/res/layout/help.xml

@@ -97,7 +97,7 @@
                 style="@style/TextMedium"
                 android:layout_width="match_parent"
                 android:layout_height="wrap_content"
-                android:text="@string/generic_no" />
+                android:text="@android:string/no" />
 
             <TextView
                 style="@style/TextMedium"

src/main/res/layout/import_activity.xml

@@ -224,7 +224,7 @@
             android:id="@+id/import_progress_right_button"
             style="@style/BottomButton"
             android:layout_gravity="end"
-            android:text="@string/generic_ok"
+            android:text="@android:string/ok"
             android:visibility="gone"
             tools:visibility="visible"/>
     </LinearLayout>

src/main/res/layout/marker_edit.xml

@@ -117,7 +117,7 @@ limitations under the License.
             android:id="@+id/marker_edit_cancel"
             style="@style/BottomButton"
             android:layout_gravity="start"
-            android:text="@string/generic_cancel" />
+            android:text="@android:string/cancel" />
 
         <Button
             android:id="@+id/marker_edit_done"

src/main/res/layout/track_edit.xml

@@ -96,7 +96,7 @@ limitations under the License.
             android:id="@+id/track_edit_cancel"
             style="@style/BottomButton"
             android:layout_gravity="start"
-            android:text="@string/generic_cancel" />
+            android:text="@android:string/cancel" />
 
         <Button
             android:id="@+id/track_edit_save"