fix(admob, android): force admob dependency to compatible v19

with the BoM it was apparently still possible for the admob library
to go across the breaking change v19->v20 boundary, this forces it to
be the specific version that works until we forward port

packages/admob/android/build.gradle

@@ -106,8 +106,7 @@ repositories {
 
 dependencies {
   api appProject
-  implementation platform("com.google.firebase:firebase-bom:${ReactNative.ext.getVersion("firebase", "bom")}")
-  implementation "com.google.firebase:firebase-ads"
+  implementation("com.google.android.gms:play-services-ads:19.8.0") { force = true; }
   implementation "com.google.android.ads.consent:consent-library:${ReactNative.ext.getVersion("ads", "consent")}"
 }