add possibility to translate strings "OK" and "Cancel" (#773)

* Update translatable.xml

* Update permission_request.xml

* Update audiopicker.xml

* Update medialibrary_preferences.xml

app/src/main/res/layout/audiopicker.xml

@@ -68,7 +68,7 @@ THE SOFTWARE.
 			android:layout_width="0px"
 			android:layout_height="wrap_content"
 			android:layout_weight="1"
-			android:text="@android:string/cancel"
+			android:text="@string/cancel"
 			android:enabled="false"
 			android:singleLine="true" />
 		<Button

app/src/main/res/layout/medialibrary_preferences.xml

@@ -125,7 +125,7 @@ along with this program. If not, see <http://www.gnu.org/licenses/>.
 			android:layout_width="wrap_content"
 			android:layout_height="wrap_content"
 			android:layout_gravity="end"
-			android:text="@android:string/cancel">
+			android:text="@string/cancel">
 		</Button>
 
 		<ProgressBar

app/src/main/res/layout/permission_request.xml

@@ -37,6 +37,6 @@ along with this program. If not, see <http://www.gnu.org/licenses/>.
 			android:clickable="false"
 			android:layout_marginRight="8dip"
 			android:layout_gravity="center"
-			android:text="@android:string/ok" />
+			android:text="@string/ok" />
 	</TableRow>
 </TableLayout>

app/src/main/res/values/translatable.xml

@@ -22,6 +22,8 @@ THE SOFTWARE.
 -->
 <resources>
 	<string name="app_name">Vanilla Music</string>
+	<string name="ok">OK</string>
+	<string name="cancel">Cancel</string>
 
 	<!-- Playback Activity (main screen) -->
 	<string name="no_songs">No tracks found on your device.</string>