Detect ANRs on Android and provide configuration option to enable detection

[fractalwrench]

Goal

Changeset

• Adds AutoDetectAnrs flag (defaulting to false) to Configuration. When this flag is altered, a method is invoked on the native interface which enables/disables ANR detection
• Adds an AutoDetectAnrs flag to the BugsnagBehaviour script, which shows up as a checkbox in the GUI
• Adds NDK + ANR crash buttons to the example app
• Updates bugsnag-android to allow for the SIGQUIT handler to be disabled: Allow disabling previous signal handler invocation for Unity ANRs bugsnag-android#743

Configuration

Configuration is now a publicly accessible interface to allow for users to supply a value for detectAnrs up-front. This also brings bugsnag-unity more inline with the notifier specification, although configuration options are still considered mutable after initialisation to remain backwards compatible.

Tests

• Verified that an ANR is detected (or not) when setting each combination of the AutoNotify/AutoDetectAnrs flags in the example app

[kattrali]

The overall intent looks good here as well as moving the interface towards spec compliance, however I raised a few inline concerns on the implementation.

addressed review comments