android: use git describe --tags --dirty for the version name

The monotonic version code is still manually bumped.

Updates tailscale/tailscale#431

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

android/build.gradle

@@ -20,13 +20,22 @@ allprojects {
 
 apply plugin: 'com.android.application'
 
+def getVersionName = { ->
+	def stdout = new ByteArrayOutputStream()
+	exec {
+		commandLine 'git', 'describe', '--tags', '--dirty'
+		standardOutput = stdout
+	}
+	return stdout.toString().trim()
+}
+
 android {
     compileSdkVersion 29
     defaultConfig {
         minSdkVersion 23
         targetSdkVersion 29
         versionCode 5
-        versionName "0.1.5"
+        versionName getVersionName()
     }
 	compileOptions {
 		sourceCompatibility 1.8