cargo2android: Fix edge case in --add_workspace feature

If the last line of a Cargo.toml was a `[*]`, then appending
`[workspace]` without newlines would result in an invalid toml file.

external/crosvm/tracing/Cargo.toml is an example.

Test: ran on crosvm
Change-Id: Ib949e7f92d0205b1bc6433d1e231924832ddb536

scripts/cargo2android.py

@@ -1360,7 +1360,7 @@ class Runner(object):
         print('### WARNING: found [workspace] in Cargo.toml')
       else:
         with open(cargo_toml, 'a') as out_file:
-          out_file.write('[workspace]\n')
+          out_file.write('\n\n[workspace]\n')
           added_workspace = True
           if self.args.verbose:
             print('### INFO: added [workspace] to Cargo.toml')