Store trace files in app-specific dirs.

The previous default location of "/sdcard" became painful to use
starting in M, because it required a runtime permission.  So now we
default to storing trace files in app-specific directories on shared
storage, which apps always have write access to with no additional
permissions.

Update docs to be consistent between all overloads.

Bug: 22807654
Change-Id: If4feca7c8778dfdf4ccce8cfb68418dc416260b5