SignApk - change signature of readPassword to use char[] instead

Summary:
Use char[] is more conventional for password handling. See this question
for reference.
https://stackoverflow.com/questions/8881291

This is to address a concern raised in
https://android-review.googlesource.com/c/platform/build/+/1890395/2

Test: mma
Change-Id: I8d60efc557d7641c057e49a2aa4613fea67cd1e6