tombstoned: Support java trace dumps.

The changes here involve :
- Creating and opening a new socket to receive trace dump requests on. Having
  different sockets allows us to install different sets of access control rules.

- A minor refactor to allow us to share common pieces of implementation
  between the java and native dumping code. This will also allow us to
  add a unit test for all file / directory related logic.

There are two java trace specific additions here :
- We use SO_PEERCRED instead of trusting the PID written to the seocket
  because requests come in from untrusted processes.
- Java trace dumps are not interceptible.

kJavaTraceDumpsEnabled is set to false for now but the value of the flag
will be flipped in a future change.

Bug: 32064548
Test: Manual; Currently working on a unit_test for CrashType.

Change-Id: I1d62cc7a7035fd500c3e2b831704a2934d725e35