JNI: use @JniType for strings in password_manager

Bug: 326465150
Change-Id: I2d515e0b962ff65e0c20d93af8245d88628ebe53
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5398678


Auto-Submit: Sam Maier <smaier@chromium.org>
Reviewed-by: Ioana Pandele <ioanap@chromium.org>
Commit-Queue: Sam Maier <smaier@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1278980}