java.security.SecureRandom: add getInstanceStrong() method

Port SecureRandom to jdk8u60.

The method getInstanceStrong returns a strong random number
generator. In Android this is assumed to be the one from
OpenSSLProvider.

This commit also contains cosmetic JavaDoc changes like
- Use of @code in place of <code> tags and use of a package-info.java in
place of a package.html file.
- Added comments.
- Uses of java.security.util.Debug (commented-out as that
debugging mechanism is not available in Android).
- Added braces in control flow blocks.

Test: make droid docs, vogar SecureRandomTest
Bug: 29631070
Change-Id: I9080fbd5e9292d7cb3eaa234f7c5849411336fac