fix(ndk): Ensure 64-bit devices running 32-bit binaries do not use 32-bit device unwinding logic

[kattrali]

Small refactor of the API 21+ stack unwinding logic to restrict 32-bit
unwinding style from 64-bit devices.

While the stack enhancements added in #378 are intended only to be run
on 32-bit devices and gated to the arm build, when 64-bit devices
are running the 32-bit library, the stacktraces for those devices were
unintentionally affected.

This is the case when an app is configured to only run 32-bit binaries
(Android cannot run both 64-bit and 32-bit binaries at once), and many
popular libraries (e.g. React Native) only distribute 32-bit libraries.

Added additional improvement to the 32-bit stacktrace resolution by
switching to nongnu libunwind only for those devices. This significantly
improves the number of frames which can be unwound without risk of a
segfault.

Tests

• Tested against ARM 32-/64-bit devices against raise(signum), abort(), floating point exceptions, and invalid casts. With the exception of abort() the stacktrace is improved.
• Sanity checked against x86/x64 to ensure the logic is restricted correctly

To reproduce the original problem add ABI filters to an apps gradle config to only load 32-bit binaries:

android {
  // ...
  defaultConfig {
    // Ensure no 64-bit libs are loaded/preferred over the 32-bit counterparts
    packagingOptions {
        exclude "lib/arm64-v8a/libbugsnag-ndk.so"
        // Add `exclude` lines for any other included library
    }

    ndk {
      abiFilters "armeabi-v7a", "x86"
    }
  }
}

Discussion

Linked issues

Fixes #382
Related to #380

Review

For the pull request reviewer(s), this changeset has been reviewed for:

• Testing additional esoteric C crashes to compare the unwinding results across devices/architectures
• Consistency between the changeset and the goal stated above
• Performance and complexity - are there any cases of unexpected O(n^3) when iterating, recursing, flat mapping, etc?
• Thoroughness of added tests and any missing edge cases
• Idiomatic use of the language

[kattrali]

Reworked the implementation a bit to:

1. Add additional inline documentation
2. Use the ABI list we already have to switch unwinding logic. Its better than linking an additional dependency.

[ellsworthrw]

This version no longer sends reports for native crashes.