Clean up BT stack log level preference controller and tests

Problem: These tests were failing because the resource ID used in the
test and its version of the R class resolved to a different integer
(and subsequently a different string array) than the R class used on
device. This was causing different string arrays with different lengths
and values to sometimes appear, especially when the source and tests
were builts separately.

Solution: Manually lookup the resource ID to use by name instead of
using the pre-baked R class and assuming its ID is correct. This allows
us to get the proper ID every time and trust the values in the array.
This trust allows for some clean up on the code and tests, such that now
our tests can iterate on values instead of relying on private constants
being made visible or even hardcoded values.

Flag: EXEMPT, test fix and minor refactor with no behavior change
Bug: 339148064
Test: atest com.android.settings.development.bluetooth.BluetoothStackLogPreferenceControllerTest
Change-Id: I8149bd06ed70589afb3797a9f453eb2a11c3c410