Ignore modifier of backing property in android_studio code style

[paul-dingemans]

Description

Ignore modifier of backing property in android_studio code style

Extract backing-property-name from property-naming rule. This allows users to disable this rule, but keep the property-naming rule in place.

For android_studio code style the restrictions regarding the modifier of the correlated property or function is ignored entirely as the Android Kotlin Styleguide does not require it to be public.

Closes #2528

Checklist

Before submitting the PR, please check following (checks which are not relevant may be ignored):

• Commit message are well written. In addition to a short title, the commit message also explain why a change is made.
• At least one commit message contains a reference Closes #<xxx> or Fixes #<xxx> (replace<xxx> with issue number)
• Tests are added
• KtLint format has been applied on source code itself and violations are fixed
• PR title is short and clear (it is used as description in the release changelog)
• PR description added (background information)

Documentation is updated. See difference between snapshot and release documentation

• Snapshot documentation in case documentation is to be released together with a code change
• Release documentation in case documentation is related to a released version of ktlint and has to be published as soon as the change is merged to master