android build: Upgrade Gradle to 7.0.2

Done to follow the template-app change in
facebook/react-native@13107fa3d. (Which it looks like RN actually
intended to make in facebook/react-native@85249cafe but forgot;
that's not important, I think.)

We didn't take facebook/react-native@13107fa3d on the path to the RN
v0.67 upgrade because of a blocker that was resolved with the Expo
44 upgrade in expo/expo@3edc37ae4. For details, expand the
`expo-screen-orientation` block in
  zulip#5203 (comment)
and see the comment on the `Installing ExpoModulesCore 0.6.4` output
from `pod install`.

Done by following the "How to upgrade Gradle" instructions at the
top of gradle-wrapper.properties. The result matched the
template-app commit except for upstream's line endings got messed
with because they didn't have Git set up properly to keep them
consistent:
  zulip#5426 (comment) .

android/gradle/wrapper/gradle-wrapper.properties

@@ -6,6 +6,6 @@
 #  the wrapper is the one from the new Gradle too.)
 distributionBase=GRADLE_USER_HOME
 distributionPath=wrapper/dists
-distributionUrl=https\://services.gradle.org/distributions/gradle-6.9-all.zip
+distributionUrl=https\://services.gradle.org/distributions/gradle-7.0.2-all.zip
 zipStoreBase=GRADLE_USER_HOME
 zipStorePath=wrapper/dists