Add __repr__ method for sample.Sample

sample.Sample has no string format method, which will cause useless
message when print log.

Change-Id: I8b9863a4ac6ee3373e18d36681fbfc4c89732111
Closes-Bug: #1386187
Fix hacking rule H305 imports not grouped correctly

The recommended group order is:

import stdlib_1
import stdlib_2

import third_party_lib_1
import third_party_lib_2

import project_lib_1
import project_lib_2

Change-Id: If2e22b25b80d6c399a872df307d7f4c58a73795f
Merge "Fix hacking rule H305 imports not grouped correctly"
Merge "Add __repr__ method for sample.Sample"