Add agent startup-attach

Add ability to give 'startup-agents' that are automatically loaded
when a (debuggable) app starts. These agents are any files in the
'code_cache/startup_agents' directory. The agents are started with the
apps data_directory as an argument.

Test: Install debuggable apk (here com.antonioleiva.bandhookkotlin)
  walleye:/ $ run-as com.antonioleiva.bandhookkotlin sh
  walleye:/data/data/com.antonioleiva.bandhookkotlin $ mkdir code_cache/startup_agents
  walleye:/data/data/com.antonioleiva.bandhookkotlin $ cp /data/local/tmp/libtifasts32.so code_cache
  walleye:/data/data/com.antonioleiva.bandhookkotlin $ cp /data/local/tmp/libtifasts64.so code_cache
  walleye:/data/data/com.antonioleiva.bandhookkotlin $ cp /data/local/tmp/libchainagentss32.so code_cache/startup_agents/
  walleye:/data/data/com.antonioleiva.bandhookkotlin $ cp /data/local/tmp/libchainagentss64.so code_cache/startup_agents/
  walleye:/data/data/com.antonioleiva.bandhookkotlin $ echo $PWD/code_cache/libtifasts32.so=log,ClassLoad > chain_agents.txt
  walleye:/data/data/com.antonioleiva.bandhookkotlin $ echo $PWD/code_cache/libtifasts64.so=log,ClassLoad >> chain_agents.txt
  Start bandhookkotlin
  Examine logcat

Bug: 135627501

Cherry-picked from commit c0fce111c8b due to merge conflict.

Change-Id: Ib82b27df90c7964a995288d8b2b3d348a11cdd80
Merged-In: Ib82b27df90c7964a995288d8b2b3d348a11cdd80