BTM-PM: Get rid of BTM_PM_DEBUG flag

* Instead of using a build time flag, use a LOG_VERBOSE
  to control the verbosity of BTM power manager logs
* Add more logging on error paths

Bug: 165181021
Test: make
Tag: #refactor
Change-Id: Iddc3cd025c2078ade4d7dd9d608a8e64b0987034