Refactor hiddenAPIEncodeDex for use by bootclasspath_fragment

Previously, if the method was called multiple times by the same module
the resulting build rules would all use the same temporary directory
which meant that if run in parallel they would conflict with each
other. This change fixes that by providing a jar specific temporary
directory so it can be used by bootclasspath_fragment to encode its
content modules.

Also, cleans up, simplifies and improves the documentation.

Bug: 179354495
Test: m com.android.art com.android.ipsec com.android.os.statsd com.android.conscrypt
      - verify that this does not change the contents of the apex files
Change-Id: I0ebe61abc8e16111c6e8a822eb96c57846b98461