[various] Plugin Gradle pre-Kotlin standardization (#11173)

As a precursor to converting plugins to use Kotlin Gradle build files in https://github.com/flutter/packages/pull/11172, this does some final standardization of the Groovy versions:
- Uses `"` instead of `'` on more lines.
- Uses `(...)` and `=` in places where it's optional, instead of just a space.
- Uses `minSdk` instead of `minSdkVersion`.
- Standardizes vertical whitespace (not necessary for the conversion, but it's something I cleaned up in that PR).

This is being done as a precursor PR because I noticed that the files are mostly right on the cusp of being similar enough for git to think they are renames with changes rather than deletion and addition; some are ending up one way, and some the other. Since renames with changes make blame much more useful, I'd like to make the files more similar so that hopefully almost all of them will end up as renames with changes in that PR.

## Pre-Review Checklist

[^1]: Regular contributors who have demonstrated familiarity with the repository guidelines only need to comment if the PR is not auto-exempted by repo tooling.