fix: be able to read more complex buildToolsVersion definitions

[dloic]

Summary:

As highlighted in https://github.com/react-native-community/cli/issues/2221, the way some projects (like expo ones) define the buildToolsVersion is breaking react-native doctor.

buildscript {
    ext {
        buildToolsVersion = findProperty('android.buildToolsVersion') ?: '34.0.0'
        minSdkVersion = Integer.parseInt(findProperty('android.minSdkVersion') ?: '23')
   ...

leads to buildToolsVersion => .b34.0.0.

The PR is slightly tweaking the parsing to account for those cases.

Test Plan:

• A new test has been added to demonstrate code is now able to properly read the buildToolsVersion
• Existing tests are already covering the basic buildToolsVersion = "a version" syntax

Checklist

• ~~[ ] Documentation is up to date to reflect these changes.~~ NA
• [x] Follows commit message convention described in CONTRIBUTING.md