chore: add CodeQL analysis (#191)

#### Details

CodeQL analysis is available through injection and this enables it
through a flag.

##### Motivation

Security requirement.

#### Pull request checklist
<!-- If a checklist item is not applicable to this change, write "n/a"
in the checkbox -->

- [.] Addresses an existing issue: #0000
- [.] Added/updated relevant unit test(s)
- [.] Ran `./gradlew fastpass` from
`AccessibilityInsightsForAndroidService`
- [x] PR title _AND_ final merge commit title both start with a semantic
tag (`fix:`, `chore:`, `feat(feature-name):`, `refactor:`).

Author: waabid

pipeline/validation-build.yaml

@@ -4,6 +4,9 @@
 trigger:
   - main
 
+variables:
+    Codeql.Enabled: true
+
 jobs:
   - job: 'gradlew_assemble'