Consolidate CI workflows and update JDK version

Removed existing Gradle build and validation workflows, replacing them
with a new unified CI configuration. Updated JDK version to 21 and added
a security workflow utilizing MobSF scan and GitHub CodeQL.

.github/workflows/main.yml

@@ -0,0 +1,33 @@
+name: Main CI
+on: [ push ]
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  spotless:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - name: Set up JDK 17
+        uses: actions/setup-java@v4
+        with:
+          distribution: temurin
+          java-version: 17
+      - name: Run Spotless
+        run: ./gradlew spotlessCheck
+
+  build:
+    runs-on: macos-14
+    steps:
+      - uses: actions/checkout@v4
+      - name: Set up JDK 17
+        uses: actions/setup-java@v4
+        with:
+          distribution: temurin
+          java-version: 17
+      - name: Setup Android problem matchers
+        uses: jonasb/android-problem-matchers-action@v1
+      - name: Build with Gradle
+        run: ./gradlew build

.github/workflows/security.yml

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+name: Security
+on: [push]
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  mobfs:
+    permissions:
+      contents: read # for actions/checkout to fetch code
+      security-events: write # for github/codeql-action/upload-sarif to upload SARIF results
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - name: Setup Python 3.10
+        uses: actions/setup-python@v4
+        with:
+          python-version: '3.11'
+      - name: Run mobsfscan
+        uses: MobSF/[email protected]
+        with:
+          args: . --sarif --output results.sarif || true
+      - name: Upload mobsfscan report
+        uses: github/codeql-action/upload-sarif@v2
+        with:
+          sarif_file: results.sarif
+
+  gradle-validate:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: gradle/wrapper-validation-action@v1