Use git commit timestamp as build timestamp.

briar-android/build.gradle

@@ -218,16 +218,16 @@ dependencyVerification {
 	]
 }
 
-def getGitHash = { ->
+def getStdout = { command, defaultValue ->
 	def stdout = new ByteArrayOutputStream()
 	try {
 		exec {
-			commandLine 'git', 'rev-parse', '--short=7', 'HEAD'
+			commandLine = command
 			standardOutput = stdout
 		}
 		return stdout.toString().trim()
 	} catch (Exception ignored) {
-		return "No commit hash"
+		return defaultValue
 	}
 }
 
@@ -243,8 +243,10 @@ android {
 		applicationId "org.briarproject.briar.android"
 		resValue "string", "app_package", "org.briarproject.briar.android"
 		resValue "string", "app_name", "Briar"
-		buildConfigField "String", "GitHash", "\"${getGitHash()}\""
-		buildConfigField "Long", "BuildTimestamp", "${System.currentTimeMillis()}L"
+		buildConfigField "String", "GitHash",
+				"\"${getStdout(['git', 'rev-parse', '--short=7', 'HEAD'], 'No commit hash')}\""
+		buildConfigField "Long", "BuildTimestamp",
+				"${getStdout(['git', 'log', '-n', '1', '--date=unix', '--format=%cd'], 0)}000L"
 	}
 
 	buildTypes {