android/gradle/wrapper: add cryptographic checksum

Suggested by F-Droid bot, https://gitlab.com/fdroid/rfp/-/issues/1546#note_443476386.

Signed-off-by: Elias Naur <mail@eliasnaur.com>

android/gradle/wrapper/gradle-wrapper.properties

@@ -1,5 +1,6 @@
 distributionBase=GRADLE_USER_HOME
 distributionPath=wrapper/dists
+distributionSha256Sum=038794feef1f4745c6347107b6726279d1c824f3fc634b60f86ace1e9fbd1768
 distributionUrl=https\://services.gradle.org/distributions/gradle-6.3-bin.zip
 zipStoreBase=GRADLE_USER_HOME
 zipStorePath=wrapper/dists