Add typed getters to DeviceConfig API.

Typed getters for String, boolean, int, double, and float. These new
methods take default_value params and return them in the event that
either a flag does not exist, or does not parse correctly.

Bug: 126415338
Test: atest FrameworksCoreTests:DeviceConfigTest
Change-Id: I10523a7b591d9f062d5de88c5fc4edd8c7c73ee0