Fix: isInstaller() for GooglePlay crashes when the passed context class is not from the root package.

Under the conditions described in the title, isInstaller() will crash with the following exception:

```java
05-20 09:13:54.851: E/AndroidRuntime(32389): Caused by: java.lang.IllegalArgumentException: Unknown package: com.something.activity
05-20 09:13:54.851: E/AndroidRuntime(32389): 	at android.os.Parcel.readException(Parcel.java:1429)
05-20 09:13:54.851: E/AndroidRuntime(32389): 	at android.os.Parcel.readException(Parcel.java:1379)
05-20 09:13:54.851: E/AndroidRuntime(32389): 	at android.content.pm.IPackageManager$Stub$Proxy.getInstallerPackageName(IPackageManager.java:2413)
05-20 09:13:54.851: E/AndroidRuntime(32389): 	at android.app.ApplicationPackageManager.getInstallerPackageName(ApplicationPackageManager.java:1118)
05-20 09:13:54.851: E/AndroidRuntime(32389): 	at org.onepf.openiab.appstore.GooglePlay.isInstaller(GooglePlay.java:67)
05-20 09:13:54.851: E/AndroidRuntime(32389): 	at org.onepf.openiab.AppstoreServiceManager.getInstallerAppstore(AppstoreServiceManager.java:146)
05-20 09:13:54.851: E/AndroidRuntime(32389): 	at org.onepf.openiab.AppstoreServiceManager.getAppstoreForService(AppstoreServiceManager.java:128)
05-20 09:13:54.851: E/AndroidRuntime(32389): 	at org.onepf.openiab.OpenIabHelper$1.onAppstoreServiceManagerInitFinishedListener(OpenIabHelper.java:85)
05-20 09:13:54.851: E/AndroidRuntime(32389): 	at org.onepf.openiab.AppstoreServiceManager.startSetup(AppstoreServiceManager.java:68)
05-20 09:13:54.851: E/AndroidRuntime(32389): 	at org.onepf.openiab.OpenIabHelper.startSetup(OpenIabHelper.java:82)
05-20 09:13:54.851: E/AndroidRuntime(32389): 	at com.something.billing.GooglePlayHelper.<init>(GooglePlayHelper.java:31)
05-20 09:13:54.851: E/AndroidRuntime(32389): 	at com.something.billing.BillingHelper.<init>(BillingHelper.java:61)
05-20 09:13:54.851: E/AndroidRuntime(32389): 	at com.something.activity.StripePreviewActivity.onCreate(SomeActivity.java:55)
05-20 09:13:54.851: E/AndroidRuntime(32389): 	at android.app.Activity.performCreate(Activity.java:5158)
05-20 09:13:54.851: E/AndroidRuntime(32389): 	at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1080)
05-20 09:13:54.851: E/AndroidRuntime(32389): 	at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2266)
05-20 09:13:54.851: E/AndroidRuntime(32389): 	... 11 more
```

which is perfectly understandable, as it checks for a subpackage being installed and not the root (app) package.

This commit fixes the problem.