Debug: Add attachJvmtiAgent with classloader

Add classloader support to android.os.Debug.attachJvmtiAgent. For
the original version without a given classloader, look up the
application's main classloader.

Bug: 65016018
Bug: 70901841
Test: m
Test: cts-tradefed run commandAndExit cts-dev
Change-Id: I649b6883e05dc2f75073fe1f978423f6a7b880df