Add CTS test for BugreportManager and BugreportParams

* Tests BugreportParams getMode() function independently.
* `am bug-report` calls BugreportManager#requestBugreport() which
internally calls BugreportManager#startBugreport().

Bug: 151111597
Bug: 151111348
Test: atest CtsBugreportTestCases
Change-Id: Id70e7828a3c82f1d422e7970a3dfca96d7b0bed6
5 files changed